controller panic after 20 times syncing sensor if user not config Spec.Escalation

[fatkun]

Describe the bug
controller panic after 20 times syncing sensor

To Reproduce
Steps to reproduce the behavior:

1. stop signal deploy
2. wait 20 times retry
3. See error

the error is produce here, It should not sendMessage if user not config Spec.Escalation.

	if err != nil {
		// now let's escalate the sensor
		// the context should have the most up-to-date version
		log.Infof("escalating sensor to level %s via %s message", ctx.s.Spec.Escalation.Level, ctx.s.Spec.Escalation.Message.Stream.Type)
		err := sendMessage(&ctx.s.Spec.Escalation.Message)
		if err != nil {
			log.Panicf("failed escalating sensor '%s'", key)
		}
	}

ERROR: logging before flag.Parse: E0810 07:30:23.167717       1 runtime.go:66] Observed a panic: &logrus.Entry{Logger:(*logrus.Logger)(0xc4203f0140), Data:logrus.Fields{}, Time:time.Time{wall:0xbed36da3c9fcf676, ext:4169274126066, loc:(*time.Location)(0x1fbbf40)}, Level:0x0, Message:"failed escalating sensor 'argo/test2'", Buffer:(*bytes.Buffer)(nil)} (&{0xc4203f0140 map[] 2018-08-10 07:30:23.16757311 +0000 UTC m=+4169.274126066 panic failed escalating sensor 'argo/test2' <nil>})
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:72
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:65
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:51
/usr/local/Cellar/go/1.10/libexec/src/runtime/asm_amd64.s:573
/usr/local/Cellar/go/1.10/libexec/src/runtime/panic.go:505
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/entry.go:126
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/entry.go:194
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/entry.go:242
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/logger.go:181
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/exported.go:155
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:125
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:182
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:175
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:133
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:134
/Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:88
/usr/local/Cellar/go/1.10/libexec/src/runtime/asm_amd64.s:2361
panic: (*logrus.Entry) (0x1533b40,0xc4213f0e10) [recovered]
        panic: (*logrus.Entry) (0x1533b40,0xc4213f0e10)

goroutine 88 [running]:
github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:58 +0x107
panic(0x1533b40, 0xc4213f0e10)
        /usr/local/Cellar/go/1.10/libexec/src/runtime/panic.go:505 +0x229
github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus.Entry.log(0xc4203f0140, 0xc420959320, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/entry.go:126 +0x2d2
github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus.(*Entry).Panic(0xc4213f0c30, 0xc4210fdce0, 0x1, 0x1)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/entry.go:194 +0xaa
github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus.(*Entry).Panicf(0xc4213f0c30, 0x157ff71, 0x1d, 0xc4210fdde0, 0x1, 0x1)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/entry.go:242 +0xed
github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus.(*Logger).Panicf(0xc4203f0140, 0x157ff71, 0x1d, 0xc4210fdde0, 0x1, 0x1)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/logger.go:181 +0x85
github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus.Panicf(0x157ff71, 0x1d, 0xc4210fdde0, 0x1, 0x1)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/github.com/sirupsen/logrus/exported.go:155 +0x5f
github.com/argoproj/argo-events/controller.(*SensorController).processNextItem(0xc420122820, 0xc4201c3200)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:125 +0x3a4
github.com/argoproj/argo-events/controller.(*SensorController).runWorker(0xc420122820)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:182 +0x2b
github.com/argoproj/argo-events/controller.(*SensorController).(github.com/argoproj/argo-events/controller.runWorker)-fm()
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:175 +0x2a
github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait.JitterUntil.func1(0xc4208dc9b0)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:133 +0x54
github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc4208dc9b0, 0x3b9aca00, 0x0, 0x1, 0x0)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:134 +0xbd
github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait.Until(0xc4208dc9b0, 0x3b9aca00, 0x0)
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:88 +0x4d
created by github.com/argoproj/argo-events/controller.(*SensorController).Run
        /Users/mmagaldi/go/src/github.com/argoproj/argo-events/controller/controller.go:175 +0x2f8

Expected behavior
It should not sendMessage if user not config Spec.Escalation.

[magaldima]

@fatkun thanks for the issue. this looks like an easy fix to just check if the configured sensor has a non nil escalation policy. I'll push up a branch with a fix!