Fixed deprecation warnings about passing null as parameter

Author: derrabus

Email.php

@@ -266,7 +266,7 @@ public function priority(int $priority)
      */
     public function getPriority(): int
     {
-        [$priority] = sscanf($this->getHeaders()->getHeaderBody('X-Priority'), '%[1-5]');
+        [$priority] = sscanf($this->getHeaders()->getHeaderBody('X-Priority') ?? '', '%[1-5]');
 
         return $priority ?? 3;
     }