Replace count with not empty

Because not all return values are countable.

Author: Evertt

src/FirebaseMessage.php

@@ -290,7 +290,7 @@ public function payload()
         }
 
         foreach (self::OPTIONS_GETTERS as $function => $arrayKey) {
-            if (method_exists($this, $function) && count($this->$function())) {
+            if (method_exists($this, $function) && !empty($this->$function())) {
                 $payload[$arrayKey] = $this->$function();
             }
         }
@@ -307,4 +307,4 @@ public function recipientNotGiven()
     {
         return !$this->recipient;
     }
-}
+}