add manifest.dash.ignoreEmptyAdaptationSet config option

[leonchen]

for #2023

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[leonchen]

@googlebot I signed it!

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[joeyparrish]

Looks good to me! I'll run the tests to verify it. Thanks for contributing!

[shaka-bot]

All tests passed!

[avelad]

I think that the PR doesn’t include demo locales.

[joeyparrish]

Oops! You're right. There's no definition for DEMO_IGNORE_DASH_EMPTY_ADAPTATION_SET. We should add that, and come up with a test to catch those mistakes before they are merged.

[joeyparrish]

I just landed a change to add the missing message. I have a test in review which would catch these mistakes in future.

[joeyparrish]

Just landed the new test as well. Thanks to @leonchen for the PR, and to @avelad for pointing out the missing message.

[joeyparrish]

This PR has been cherry-picked for v2.5.8.