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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@418641 f38db490-d61c-443f-a65b-d21fe96a405b