Some ISDN switches complain or block the call if the RDNIS number is
empty.
* Made chan_iax2 not save a RDNIS number into the ast_channel if the
string is blank. This is what other channel drivers do.
(closes issue ASTERISK-17152)
Reported by: rmudgett
git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@348735 65c4cc65-6c06-0410-ace0-fbb531ad65f3