DNS: Fix some corner cases.

* Fix query_set destruction before we are done kicking the queries off.

* Fixed no queries requested handling.

* Add empty queries request unit test.

* Added missing allocation check in ast_dns_query_set_add().

* Made initial pjsip resolving query vector slightly larger.

ASTERISK-25115
Reported by: John Bigelow

Change-Id: Ie8be8347d0992e93946d72b6e7b1299727b038f2