fix(frontend): allow null/undefined to pass enum validation for optional operator properties

[rbelavadi]

What changes were proposed in this PR?

Fix enum validator for optional operator properties.

Currently, the validator uses c.value ?? "" which coerces undefined to
empty string, then checks if "" is in the enum. Since empty string is
never a valid enum option, optional fields left blank always fail validation
and show a spurious error to the user.

Replace with c.value == null check — if the value is unset (null or
undefined), the field passes validation. Only run the enum check if the
user has actually entered a value.

Before: c.value ?? "" — coerces no-input to empty string, always fails
After: c.value == null || mapSource.enum?.includes(c.value) — allows unset values

Any related issues, documentation, discussions?

Fixes #3524

How was this PR tested?

Manually verified the logic change:

• undefined == null evaluates to true, so unset optional fields now pass validation
• A field with an actual value still gets checked against the enum
• Existing test suite passes

Was this PR authored or co-authored using generative AI tooling?

Co-authored using Claude (Anthropic). The fix was reviewed and verified by the author.

[github-actions]

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