https://sonarly.com/issue/32512?type=bug
The workflow filter step's `evaluateRelationFilter` function throws an unhandled error when a relation filter uses the `IS_NOT_NULL` operand, causing the entire workflow run to fail.
Fix: Added `ViewFilterOperand.IS_NOT_NULL` as a handled case in `evaluateRelationFilter`, falling through to `IS_NOT_EMPTY` behavior (`return isNonEmptyString(leftValue)`). This is the correct semantic: IS_NOT_NULL for a relation field means "the relation exists / has a value", identical to IS_NOT_EMPTY.
Added two test cases covering IS_NOT_NULL on RELATION filters:
- Returns true when the left operand is a non-empty string (relation exists)
- Returns false when the left operand is empty (relation does not exist)