LibWeb/CSS: Make it clear that StyleSheet::media() is never null

Author: https://github.com/AtkinsSJ
Commit: 8beade51e0
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4285
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/CSS/StyleSheet.h

@@ -36,7 +36,7 @@ public:
 
     void set_type(String type) { m_type_string = move(type); }
 
-    MediaList* media() const
+    GC::Ref<MediaList> media() const
     {
         return m_media;
     }