Clarify condition

I want to make it a little more clear that we are checking exactly
for these two values. That may also help preventing further confusion
as to why we are not using empty() here.

Amendment to PR #1033.

src/Core/Command/RegisterUserHandler.php

@@ -116,7 +116,7 @@ class RegisterUserHandler
         // from the get-go.
         if (isset($token)) {
             foreach ($token->payload as $k => $v) {
-                if ($user->$k === '' || ! isset($user->$k)) {
+                if (in_array($user->$k, ['', null], true)) {
                     $user->$k = $v;
                 }
             }