dash: Do not request undeclared initialization segments

[peaBerberian]

While fixing some integration tests of #1884, I noticed that we had poor handling of some DASH contents without an initialization segment.

For some, instead of skipping it, we still requested a URL corresponding to the BaseURL.

To fix it I clarified the semantics of the private initialization state in indexes (SegmentList, SegmentTemplate, SegmentTimeline...).

Previously it was optional, and potentially undefined but the undefined case's semantics was never documented. Now I propose to make it null when not set and explicitly document that it means that this Representation has no initialization segment available.

[github-actions]

✅ Automated performance checks have passed on commit 16697534f8a990b113fd13928b76db6960f1d553 with the base branch dev.

Details

Performance tests 1st run output

No significative change in performance for tests:

Name	Mean	Median
loading	25.49ms -> 25.48ms (0.019ms, z: 0.09675)	35.25ms -> 35.25ms
seeking	190.33ms -> 187.50ms (2.830ms, z: 1.03482)	18.75ms -> 18.75ms
audio-track-reload	31.61ms -> 31.61ms (0.006ms, z: 0.19677)	46.80ms -> 46.80ms
cold loading multithread	54.19ms -> 51.15ms (3.041ms, z: 12.09252)	76.65ms -> 75.45ms
seeking multithread	86.88ms -> 90.30ms (-3.424ms, z: 0.70427)	19.80ms -> 19.95ms
audio-track-reload multithread	30.94ms -> 30.85ms (0.092ms, z: 0.53130)	45.15ms -> 45.15ms
hot loading multithread	20.70ms -> 22.04ms (-1.341ms, z: 0.49070)	30.15ms -> 30.15ms

[github-actions]

✅ Automated performance checks have passed on commit 3cec903e5ae04c56eeb1babdb78ee5980e311e92 with the base branch dev.

Details

Performance tests 1st run output

No significative change in performance for tests:

Name	Mean	Median
loading	27.82ms -> 31.31ms (corrected: -3.555ms, A/A bias: 0.064ms, z: 0.60622)	22.95ms -> 23.10ms (corrected: -0.250ms, A/A bias: 0.150ms, z: 1.36640)
seeking	470.99ms -> 500.92ms (corrected: -67.567ms, A/A bias: 37.633ms, z: 1.41451)	11.75ms -> 17.40ms (corrected: -1.450ms, A/A bias: 1.000ms, z: 1.61658)
audio-track-reload	22.67ms -> 22.79ms (corrected: -0.274ms, A/A bias: 0.155ms, z: 0.77461)	22.00ms -> 22.15ms (corrected: -0.275ms, A/A bias: 0.250ms, z: 1.76092)
loading over active content	15.53ms -> 15.84ms (corrected: 0.108ms, A/A bias: -0.412ms, z: 0.29830)	14.55ms -> 14.25ms (corrected: -0.125ms, A/A bias: 0.050ms, z: 0.29830)
large multi-period manifest	65.77ms -> 65.23ms (corrected: -1.927ms, A/A bias: 2.464ms, z: 0.21170)	59.85ms -> 58.60ms (corrected: -2.200ms, A/A bias: 3.875ms, z: 0.21170)
cold loading multithread	38.12ms -> 37.79ms (corrected: 2.826ms, A/A bias: -2.489ms, z: 2.62213)	37.20ms -> 36.60ms (corrected: 0.775ms, A/A bias: -0.150ms, z: 3.28609)
seeking multithread	831.77ms -> 843.98ms (corrected: -14.626ms, A/A bias: 2.421ms, z: 0.21170)	1005.50ms -> 1005.60ms (corrected: 0.025ms, A/A bias: 0.050ms, z: 0.44745)
audio-track-reload multithread	23.17ms -> 22.98ms (corrected: 0.163ms, A/A bias: 0.023ms, z: 1.29423)	22.90ms -> 22.70ms (corrected: 0.400ms, A/A bias: -0.050ms, z: 2.22761)
hot loading multithread	14.49ms -> 14.31ms (corrected: 0.153ms, A/A bias: 0.021ms, z: 0.84197)	14.20ms -> 14.10ms (corrected: -0.050ms, A/A bias: 0.025ms, z: 0.66876)