func_uri: URIENCODE/URIDECODE - allow empty strings as argument

func_uri: URIENCODE/URIDECODE - allow empty strings as argument

Previously these two dialplan functions would issue warnings and
return failure when an empty string is used as the argument. Now
they will not issue a warning and will successfully return an
empty string.

ASTERISK-23911 #close
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/3745/
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Merged revisions 418641 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 418649 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@418650 65c4cc65-6c06-0410-ace0-fbb531ad65f3