🐛(frontend) only show the effect button when the track is defined

Guard the effect button so it only renders when the track exists.
This prevents the frontend build from failing when TypeScript
rightly flagged the possibility of an undefined track being passed
to the effect logic.

src/frontend/src/features/rooms/components/Join.tsx

@@ -394,7 +394,7 @@ const VideoPreview = ({
           />
         </div>
         <div className={styles.effectsOverlay}>
-          <Effects videoTrack={videoTrack} />
+          {videoTrack && <Effects videoTrack={videoTrack} />}
         </div>
       </div>
     </div>