Entity draft 3275968 3275968 invalidargumentexception string when
requested to merge issue/entity_draft-3275968:entity_draft-3275968-3275968-invalidargumentexception-string-when into 2.0.x
- Fix "InvalidArgumentException: $string": when using "Submit message";
- Fix error when $entity_draft_message['value'] is empty;
- Fix entity_draft.info.yml yml-schema;