Bug 1733034 - Fix unreachable-code-return warning in dom/media. r=padenot

dom/media/mediacontrol/MediaStatusManager.cpp:260:13: error: 'return' will never be executed [-Werror,-Wunreachable-code-return]
  return u""_ns;
            ^~~

Differential Revision: https://phabricator.services.mozilla.com/D126871

dom/media/mediacontrol/MediaStatusManager.cpp

@@ -256,8 +256,9 @@ nsString MediaStatusManager::GetDefaultFaviconURL() const {
     return u""_ns;
   }
   return NS_ConvertUTF8toUTF16(spec);
-#endif
+#else
   return u""_ns;
+#endif
 }
 
 void MediaStatusManager::SetDeclaredPlaybackState(