Fix chan_iax2 to not report an RDNIS number if it is blank.

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
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Merged revisions 348735 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 348736 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348737 65c4cc65-6c06-0410-ace0-fbb531ad65f3