The current implementation of `JpegMotionPhotoExtractor.sniff` returns
`true` for any image with Exif data (not just motion photos). Improving
this is tracked by b/324033919. In the meantime, when we 'extract' a
non-motion photo with `JpegMotionPhotoExtractor`, the result is
currently a single empty image track and no video track (since there's
no video, since this isn't a motion photo). This 'empty' image track
is usually used to transmit metadata about the video parts of the
image file (in the form of `MotionPhotoMetadata`), but this metadata is
also (understandably) absent for non-motion photos. Therefore there's
no need to emit this image track at all, and it's clearer to emit no
tracks at all when extracting a non-motion photo using
`JpegMotionPhotoExtractor`.
This change also removes a `TODO` that is misplaced, since there's no
image bytes being emitted here (and never was).
PiperOrigin-RevId: 604688053
7ae3d69e00