Handle nullable date in Clock::at() - cleanup

src/Ouzo/Goodies/Utilities/Clock.php

@@ -64,7 +64,7 @@ public static function now(): Clock
     /** Obtains a Clock set to to a specific point. */
     public static function at(?string $date): Clock
     {
-        $date = $date ?? Strings::EMPTY;
+        $date ??= Strings::EMPTY;
         $dateTime = new DateTime($date);
         return new Clock($dateTime);
     }