Fixed ConditionalAddTextStringToMap

Fixed ConditionalAddTextStringToMap

Refactoring changes the function to return an error in the 'null'
condition rather than remain a valid value.  This patch returns it to
its previous behavior by having it return 0

Change-Id: I7a3b42399dec2df3955f2b4ef513fa0ca7433bb8
Signed-off-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2212
Reviewed-by: Sakthivel Samidurai <sakthivel.samidurai@intel.com>
Reviewed-by: Omkar Hegde <omkar.m.hegde@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>