Set 'EnumControl' property values to 'null' in properties JSON

[kiersten-stokes]

Related to elyra-ai/pipeline-editor#186

What changes were proposed in this pull request?

Changes the default value of runtime_image to null in the generic properties template. This property is our only recurring EnumControl property for generic components. For custom components, we haven't run into a situation where EnumControl is used, but I've added a simple if/else block to the custom component properties template to account for that situation should it show up in the future.

How was this pull request tested?

Updates one relevant backend test resource.
Updates one relevant integration test snapshot resource.

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[akchinSTC]

looks good!

[ajbozarth]

Tested locally and this LGTM, my only question is should we cut the pipeline-editor release and update it here or merge this and use a follow up to cut release and update it closer to elyra 3.6 final release? (opinions @akchinSTC ?)

[kiersten-stokes]

Tested locally and this LGTM, my only question is should we cut the pipeline-editor release and update it here or merge this and use a follow up to cut release and update it closer to elyra 3.6 final release?

I have no preference! Probably easiest just to do it here if no objections