fix: Fix playback stuck on initial gap

[vlazh]

Fixes #6339

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[shaka-bot]

Incremental code coverage: 100.00%

[joeyparrish]

@vlazh, thank you for the fix! Could you try to add a test for this? It would be nice to know right away if we ever reintroduced this bug in a future refactor.

Since the fix is in lib/player.js, it might be best to test for it in test/player_integration.js or test/player_unit.js. Let me know if you need any guidance on how to do that.

[vlazh]

@joeyparrish could you review again?

[joeyparrish]

I double-checked that the test fails with the fix commented out, so it is an effective test. Thanks!