Don't perform a realtime lookup with a NULL keyword

Previously, a call to ast_load_realtime_multientry could get away with
passing a NULL parameter to the function, even though it really isn't
supposed to do that. After the change over to using ast_variable instead
of variadic arguments, the realtime engine gets unhappy if you do this.

This was always an unintended function call in app_directory anyway - now,
we just don't call into the realtime function calls if we don't have anything
to query on.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388008 65c4cc65-6c06-0410-ace0-fbb531ad65f3