app_queue.c: Fix json ref leak

Declining the queue_member_status_type stasis message in stasis.conf
causes these messages to leak json objects.

* Add missing ast_json_unref() if the type is NULL in
queue_publish_member_blob().

ASTERISK-28084

Change-Id: I691ecf49bd1f7d9c29182e1eee8c4bb7103be9fc