Return NULL when no results returned for realtime_multientry

Return NULL when no results returned for realtime_multientry

It was not documented what the return value should be when no entries
were returned with the multientry realtime callback. This change forces
consistent behavior even if the backends return an empty ast_config.

Review: https://reviewboard.asterisk.org/r/1521/
........

Merged revisions 342223 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@342224 65c4cc65-6c06-0410-ace0-fbb531ad65f3