Update powerwall.sh

powerwall.sh

@@ -17,7 +17,7 @@ GETTEMP(){
 	mqtt_message=`mosquitto_sub -h 10.10.1.180 -t tasmota/discovery/${SENSOR_MQTTID[$1]}/sensors -u homeassistant -P 12home34assistant56 -C 1`
 	echo $mqtt_message | cut -d':' -f 7 | cut -d',' -f 1 | read temp_c
  	echo "'$temp_c'"
-	if [ "$temp_c" != null ]; then
+	if [ "$temp_c" != "null" ] &&  "$temp_c" != "" ]; then
 		temp_f=`echo "scale=2; $temp_c*1.8 + 32" | bc`
 		echo "$temp_c -> $temp_f"
 	else