test: cover reflect.Func branch in isEmptyValue

Struct fields with function types were never exercised, leaving the
reflect.Func case in isEmptyValue uncovered. Add TestMergeFuncField to
verify that a nil func field is treated as empty (filled from src) and a
non-nil func field is treated as non-empty (not overwritten without
WithOverride).

Closes #303

Co-authored-by: Ona <no-reply@ona.com>