gh-116738: Align bz2 unused_data getter to lzma implementation

[yoney]

This is a follow-up to #142947 to align bz2 with lzma implementations by returning b"" from unused_data getter when eof is not set (doc). It also refactors the unused_data to make it easier to read and to clearly show that self->unused_data cannot be NULL (since PyBytes_FromStringAndSize() only fails on allocation errors).

Ran the test with tsan build.

cc: @emmatyping @colesbury

• Issue: Audit all built-in modules for thread safety #116738

[gpshead]

thanks! (skipping a news entry as this doesn't change anything meaningfully observable, only a generally implausible to code for case with a MemoryError buried in the midst to the circumstances)

[miss-islington-app]

Thanks @yoney for the PR, and @gpshead for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @yoney and @gpshead, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker afbe1373ac9ac96149936211b2df71c2b6f97a4b 3.14

[gpshead]

not worrying about the backport, that was just to see if it was easy.