Fixed CSRF-null bug

A bug caused FuzeWorks instances to crash when CSRF protection was disabled.

This was fixed by allowing the method to return null as well.

src/FuzeWorks/Security.php

@@ -320,7 +320,7 @@ class Security {
      * @see		Security::$_csrf_hash
      * @return 	string	CSRF hash
      */
-    public function get_csrf_hash(): string
+    public function get_csrf_hash(): ?string
     {
         return $this->_csrf_hash;
     }