Add validation for edge JS expressions

[theroinaochieng]

User story

As a workflow developer, I want validation for blank JS expressions on edges.

Implementation notes

Validation requirements:

• Cannot be blank when js_expression selected

[doc-han]

I just picked this issue and for some unknown reason I'm thinking of all the several ways an exploit could be carried out via this small beautiful looking input box. I don't think this box is near safe at the moment.