LibWeb: Guard MediaQueryList event listener removal against null

A recently imported WPT test has a subtest that effectively does the
following:

    const mql = window.matchMedia("");
    mql.removeListener(null);

Libraries/LibWeb/CSS/MediaQueryList.cpp

@@ -99,6 +99,7 @@ void MediaQueryList::remove_listener(JS::GCPtr<DOM::IDLEventListener> listener)
     // 1. Remove an event listener from the associated list of event listeners, whose type is change, callback is listener, and capture is false.
     // NOTE: While the spec doesn't technically use remove_event_listener and instead manipulates the list directly, every major engine uses remove_event_listener.
     //       This means if an event listener removes another event listener that comes after it, the removed event listener will not be invoked.
+    if (listener)
         remove_event_listener_without_options(HTML::EventNames::change, *listener);
 }