[ticket/13337] Allow empty array passed to sql_in_set()

is_array() will evaluate to true even if the array is just an empty array.
Make sure we also expect an empty array passed to sql_in_set().

PHPBB3-13337
Merge pull request #3144 from marc1706/ticket/13337

[ticket/13337] Do not use SQL IN comparison with empty array
Merge branch 'develop-ascraeus' into develop