Address #358: Only process valid connections

[cameronbrunner]

The _from_server_socket routine may return a None which should not be sent to workers to process.

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

#358

❓ What is the current behavior? (You can also link to an open issue here)

#358

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

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This change is 

[webknjaz]

@liamstask what's your opinion on this?

[webknjaz]

@cameronbrunner this looks like a simple change but, as you've demonstrated, missing tiny details may lead to disastrous server states. So could you please add a pytest-based regression test verifying this behavior?

[liamstask]

Seems reasonable to me as an immediate fix for this issue, though agreed we need a regression test.

Would also be good to add a docstring/comment to _from_server_socket() documenting this behavior.

This behavior may also explain the unresponsive behavior, as a result of SSL errors, reported in #346

For separate follow up:

• thread pool should also likely use _SHUTDOWNREQUEST = object() to create a dedicated sentinel rather than reusing None
• _from_server_socket() is generally ripe for improvement and refactoring

[webknjaz]

I'm merging this as a hotfix and I hope improvements will get added over time too.

[webknjaz]

Released as v8.5.2 @ https://pypi.org/project/cheroot/8.5.2/