[ticket/11507] Allow the posts awaiting approval value to be 0.

PHPBB3-11507
[ticket/11507] Fix the unapproved posts count query.

Only unapproved posts should be counted.

PHPBB3-11507
[ticket/11507] Allow group & unapproved post count options to be used alone.

PHPBB3-11507
[ticket/11507] Handle empty sets correctly.

Using the last parameter in $db->sql_in_set() does not give the correct
behaviour expected here. The query should not be negated if the set is empty.

PHPBB3-11507
Merge remote-tracking branch 'prototech/ticket/11507' into develop

* prototech/ticket/11507:
  [ticket/11507] Handle empty sets correctly.
  [ticket/11507] Allow group & unapproved post count options to be used alone.
  [ticket/11507] Fix the unapproved posts count query.
  [ticket/11507] Allow the posts awaiting approval value to be 0.