Update users.php

Fixes an unknown `uid` column on insert MySQL error migrating Joomla 1.5 -> 3.3
added an isset check

I don't think version granularity is super-important at this point, UID is a legacy piece of useless info, if it is set, we can safely unset it.
Merge pull request #12 from bobbravo2/patch-1

Fixes an unknown uid column on insert MySQL error migrating Joomla 1.5 -> 3.3. Thanks @bobbravo2