Make the video behavior less confusing

There's no reason to allow it to take on placeholder values. It should be defined when the media has a published video track and undefined when not.

src/tile/GridTile.tsx

@@ -150,7 +150,7 @@ const UserMediaTile: FC<UserMediaTileProps> = ({
   const tile = (
     <MediaView
       ref={ref}
-      video={video ?? undefined}
+      video={video}
       userId={vm.userId}
       unencryptedWarning={unencryptedWarning}
       encryptionStatus={encryptionStatus}