Add test-only coverage for behavior that landed during the nine-way split but was left unverified:
- Term select resolves to the configured #depth when the options(depth:) argument is omitted (new term_select_no_depth.gql + TermSelectTest case).
- A [date:*] token in an element #description forces response max-age=0 (new TimeTokenCacheabilityTest + graphql_webform_test_desc_token fixture).
- Empty-string submissions are accepted as "no value" across the text, number, telephone, date, datetime and datelist families (new FormSubmissionEmptyStringTest).
Closes #3602639