Allow disabling all alpha optimizations when used as a library

[drojf]

I was using oxipng as a library, and wanted to disable all alpha optimizations (to be fully lossless). I setting the alphas field of the Option struct to an empty hashset, but I hit an assert in the library.

I didn't see any reason why the assert should be there, so I removed the assert, and made the function apply no processing if the hashset was empty. I didn't encounter any problems converting some files (they came out as identical), and cargo test passed for me.

I've made this pull request just in case this assert is no longer needed. If it's actually there for good reason you can close this pull request.

[shssoichiro]

This does seem like a bug. In fact, I'd like to go further and go along with the idea that the default should be to not optimize alphas. That can be done in a separate PR. I'll merge this for now to get your workflow unblocked. Thanks!