[warm-reboot]: Check empty key before issuing redis hget (#1496)

Added a check for empty key before checking WARM_RESTART_ENABLE_TABLE

Author: vaibhavhd

scripts/fast-reboot

@@ -392,7 +392,7 @@ function save_counters_folder() {
 
 function check_warm_restart_in_progress() {
     sonic-db-cli STATE_DB keys "WARM_RESTART_ENABLE_TABLE|*" | while read key ; do
-        if [[ x"$(sonic-db-cli STATE_DB hget $key enable)" == x"true" ]]; then
+        if [ -n "$key" ] && [[ x"$(sonic-db-cli STATE_DB hget $key enable)" == x"true" ]]; then
             if [[ x"${FORCE}" == x"yes" ]]; then
                 debug "Ignoring warm restart flag for ${key#*|}"
             else