Fix Trigger UI form rendering for null enum values

[Subham-KRLX]

This PR fixes a regression where the Trigger UI form crashes if a parameter's enum contains null (used to make fields optional).

The zag-js library (used by Chakra UI's Select component) requires string values for dropdown items. When null is passed in an enum, it causes a crash with [zag-js] No value found for item {"label":null,"value":null}.

This fix:

1. In FieldDropdown.tsx, null enum values are converted to the string "null" specifically for the UI rendering.
2. When the value changes, the string "null" is converted back to the actual null primitive in the form state.
3. labelLookup is updated to safely handle null keys.

I've added comprehensive unit tests in FieldDropdown.test.tsx covering:

• Rendering mixed enums with null
• Custom labels for null via values_display
• Clearing selections
• Form submission correctness

closes: #62049

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[jscheffl]

Static checks are failing most probably you should install prekand run static checks locally. What I see as compliaints can all be auo-fixed.

[jscheffl]

It seems the values display for null is not working, it always displays nullas literal:

[jscheffl]

I think the replacement is a good idea such that the used library does not fail. But in my view the literal null looks a bit dangerous as I assume it might be really used as a value / string.

Would it be possible to rather switch to some exotic string with less potential for clash like __null__ or __#null#__? Woul dbe good to define this as constant that it is used consistently in code.

[Subham-KRLX]

I have updated the implementation to use a constant null internally to avoid potential clashes with literal string values. I have also fixed the values_display lookup logic to correctly handle None keys (by adding a fallback check for 'None'). Additionally I ran a full suite of static checks (prek/eslint) to fix all formatting and linting issues.