unqiueid: correct max uniqueid length test

This patch adds null string test prior to checking for
a max uniqueid value that was added in r410157.
........

Merged revisions 410368 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410369 65c4cc65-6c06-0410-ace0-fbb531ad65f3