Stop trying to put null strings in the UUID cache

Before, the server would try to place the UUID string into the userLookup database even if the String was null, causing a NullPointerException.  This fix verifies that there is an actual UUID to be stored before attempting to put it in the database, and, if there is no UUID to be found, returns null instead of causing an Exception
Merge pull request #32 from techkid6/patch-1

Stop trying to put null strings in the UUID cache