Fix Mp4Extractor OutOfBounds exception when sample tables are omitted

When `FLAG_OMIT_TRACK_SAMPLE_TABLE` is set, `Mp4Extractor` initializes
per-sample arrays (like `timestampsUs`) with length 0 to save memory,
but the total `sampleCount` remains greater than 0.

If an MP4 file lacks an `stss` box, all frames are considered sync
samples. When scanning for a thumbnail or resolving seek points, the
extractor limits its search to the number of sync samples. In this
case, that limit becomes the total `sampleCount`.

Because the extractor attempts to loop up to `sampleCount` and read from
the 0-length `timestampsUs` array, it throws an
`ArrayIndexOutOfBoundsException`.

This change fixes the issue by adding `hasSampleTableData()` checks to
`findBestThumbnailPresentationTimeUs` and `TrackSampleTable` search
methods to return early when the per-sample mapping arrays are empty.

The test file was generated from the existing `sample.mp4` asset. To
ensure it lacks an `stss` box, every video frame was forced to be a
keyframe (GOP size 1). The audio track was also removed to reduce file
size.

`$ ffmpeg -i sample.mp4 -c:v libx264 -g 1 -an sample_without_stss.mp4`

PiperOrigin-RevId: 908211794
(cherry picked from commit 802042e3da151a404f8bf7753c3935edc1cff764)