Fixed PlayerSetModel job field causing errors when a null value is returned

This prevented implementation of PlayerSetModel for only a specific condition (eg. VIP models).

Fixes http://forum.darkrp.com/threads/custom-job-field.439/page-2#post-1782
Fixed F4 choose model panel incorrectly hiding when PlayerSetModel returns null
Merge pull request #1180 from Bo98/playersetmodel-fix

Fixed PlayerSetModel job field causing errors when a null value is returned