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: http://svn.asterisk.org/svn/asterisk/trunk@410369 f38db490-d61c-443f-a65b-d21fe96a405b