SlackStatus: Prevent error output when expiry is empty

This can cause issues if the user wants to parse the server reponse

src/set-slack-status/default.sh

@@ -19,7 +19,7 @@ if [[ ! -z "$KMPARAM_Status_text" || $both_empty -eq 0 ]]; then
 fi
 
 # Calculate expiration unixtime (if set to more than 0)
-if [[ $(bc -l <<< "$KMPARAM_Clear_after_x_hours > 0") -eq 1 ]]; then
+if [[ ! -z "$KMPARAM_Clear_after_x_hours" && $(bc -l <<< "$KMPARAM_Clear_after_x_hours > 0") -eq 1 ]]; then
  unix_now=$(date +%s)
  offset=$(bc -l <<< "$KMPARAM_Clear_after_x_hours * 3600")
  integer_offset=$(printf "%.0f" $offset)