Clear out workflowDefinition in subworkflow param if it was touched t…

[ajlai-orkes]

…hen cleared

Pull Request type

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• WHOSUSING.md
• Other (please describe):

NOTE: Please remember to run ./gradlew spotlessApply to fix any format violations.

Changes in this PR

• Fix Sub Workflow task editor so an empty Workflow definition field is omitted from subWorkflowParam instead of being saved as "" or {}.
• Empty values were failing AJV validation (must be string / anyOf) and blocking save until the field was manually removed in the Code tab.
• Real string references and inline workflow objects are still written when provided.

Essentially clearing the workflowDefinition in subworkflow was keeping this around which the validation would fail

   "subWorkflowParam": {
        "workflowDefinition": ""
      },

Before fix:
https://www.loom.com/share/b7af54a791e34812815d8ef2e6378970

After fix:
https://www.loom.com/share/6ac41b2501c94bf48e4945222a4bbe47

[nthmost-orkes]

if it works!