cautious treatment of compiler in b2 generator

[grisumbras]

Using settings.get_safe more actively. Conversion of Nones to strings.
Fixes #4201

Changelog: Bugfix: b2 generator was failing when package recipe didn't use compiler setting
Docs: omit

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• I've followed the PEP8 style guides for Python code.

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[danimtb]

Thanks for the simplification! We are willing to review the whole b2 generator so this is very welcome 😄

[danimtb]

Hi @grisumbras!

Thanks for your PR. Could you please include a brief changelog in the description of this PR? This will help us to include this in the next release. Thanks! 😃

[danimtb]

Oh, I forgot to mention! It would be nice to write a test with the use case that was failing before this changes. I could help with it in case you are not familiar with them or don't have time. Just ping me!

[grisumbras]

Should I prepend a commit with test or just add a commit?

[danimtb]

An additional commit will be fine. We mostly review the diff so it doesn't matter that much

[grisumbras]

Also, how should I create a ConanFile instance suitable for constructing a generator?

[danimtb]

Have a look at https://github.com/conan-io/conan/blob/develop/conans/test/unittests/client/generators/b2_test.py

I guess your test should be very similar

[grisumbras]

I updated the PR description and added a test.

[danimtb]

Cool! Thanks @grisumbras

[lasote]

@danimtb please add the changelog and docs entries to this PR body.

[lasote]

My bad, it was already there.