fix(telegrafreceiver): make shutdown safe to call before start

[swiatekm]

Shutdown can be called without Start having been called before. In particular, this happens if there's an error starting a component in the pipeline before receivers are started. We then get a segfault:

2023-01-30T12:49:29.788+0100    info    service/pipelines.go:121        Stopping receivers...
2023-01-30T12:49:29.788+0100    info    telegrafreceiver/receiver.go:156        Stopping telegraf receiver      {"kind": "receiver", "name": "telegraf", "pipeline": "metrics"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x2f7cd0f]

goroutine 1 [running]:
github.com/SumoLogic/sumologic-otel-collector/pkg/receiver/telegrafreceiver.(*telegrafreceiver).Shutdown.func1()
        github.com/SumoLogic/sumologic-otel-collector/pkg/receiver/telegrafreceiver/receiver.go:157 +0x4f
sync.(*Once).doSlow(0xc9057e?, 0x7000000087044ad?)
        sync/once.go:74 +0xc2
sync.(*Once).Do(...)
        sync/once.go:65
github.com/SumoLogic/sumologic-otel-collector/pkg/receiver/telegrafreceiver.(*telegrafreceiver).Shutdown(0x7f00e0?, {0x62e7db5?, 0x2?})
        github.com/SumoLogic/sumologic-otel-collector/pkg/receiver/telegrafreceiver/receiver.go:155 +0xd5
go.opentelemetry.io/collector/service.(*builtPipelines).ShutdownAll(0xc001565400, {0x6f71400, 0xc000194000})
        go.opentelemetry.io/[email protected]/service/pipelines.go:124 +0x499
go.opentelemetry.io/collector/service.(*Service).Shutdown(0xc000925680, {0x6f71400, 0xc000194000})
        go.opentelemetry.io/[email protected]/service/service.go:155 +0xd4
go.opentelemetry.io/collector/otelcol.(*Collector).setupConfigurationComponents(0xc0014bfaa8, {0x6f71400, 0xc000194000})
        go.opentelemetry.io/[email protected]/otelcol/collector.go:181 +0x568
go.opentelemetry.io/collector/otelcol.(*Collector).Run(0xc0014bfaa8, {0x6f71400, 0xc000194000})
        go.opentelemetry.io/[email protected]/otelcol/collector.go:205 +0x65
go.opentelemetry.io/collector/otelcol.NewCommand.func1(0xc0001fc000, {0x6296a87?, 0x1?, 0x1?})
        go.opentelemetry.io/[email protected]/otelcol/command.go:53 +0x419
github.com/spf13/cobra.(*Command).execute(0xc0001fc000, {0xc0001a2010, 0x1, 0x1})
        github.com/spf13/[email protected]/command.go:916 +0x862
github.com/spf13/cobra.(*Command).ExecuteC(0xc0001fc000)
        github.com/spf13/[email protected]/command.go:1044 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/[email protected]/command.go:968
main.runInteractive({{0xc000e32ed0, 0xc000e33f50, 0xc000e33230, 0xc000e32b70}, {{0x62acd07, 0xc}, {0x63bbff1, 0x2f}, {0x632f679, 0x1d}}, ...})
        github.com/SumoLogic/sumologic-otel-collector/main.go:37 +0x7d
main.run(...)
        github.com/SumoLogic/sumologic-otel-collector/main_others.go:11
main.main()
        github.com/SumoLogic/sumologic-otel-collector/main.go:25 +0x1d8