When the field is blank, don't warn about the field being unable to be...

When the field is blank, don't warn about the field being unable to be coerced, just skip the column.
(closes http://lists.digium.com/pipermail/asterisk-dev/2009-December/041362.html)
Reported by Nic Colledge on the -dev list, fixed by me.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@236847 65c4cc65-6c06-0410-ace0-fbb531ad65f3