Support keyword nil in expression

[lixingwang]

I'm submitting a ... (check one with "x")

[] bug report => search github for a similar issue or PR before submitting
[x] enhancement request
[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
Today, there is no way to check whether the property's value is empty or it non-exist.
In order to support this case. we plan to support != nil expression.
Expected behavior:
Introduce a keyword nil in expression, it representative empty value, just like Golang nil.
Such as $activity[rest].content.id !=nil or activity[rest].content.id == nil
Minimal steps to reproduce the problem (not required if feature enhancement):

What is the motivation / use case for changing the behavior?

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

Additional information you deem important (e.g. issue happens only occasionally):