Treat empty format selection string as "anything goes".

[mk-pmb]

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Empty format selection string always leads to ExtractorError: requested format not available, even if there clearly are some. If the user has no criteria at all, i.e. anything goes, then worst obviously is good enough for them, but a lie is not.

I had considered giving them best, but for users who really don't care about media quality, we should optimize for low power consumption in all affected equipment.

[dirkf]

But there's a default if the option isn't mentioned.

Is this an API issue?

[mk-pmb]

Yes, it is an API issue. What I really wanted to do was submit another test case, but to express that other test case in a simple way, I need this patch here first. Otherwise I'd have to include a workaround for this issue here and explain the seeming lie.