Don't add an empty MESSAGE_DATA(key) header if it doesn't already exist.

Doing Set(MESSAGE_DATA(key)=) would add an empty key header if the key
header did not already exist.  If it already existed it would delete it.

* Made msg_set_var_full() exit early if the named variable did not already
exist and the value to set is empty.
........

Merged revisions 361522 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@361523 65c4cc65-6c06-0410-ace0-fbb531ad65f3