Treat fd=0 as a valid file descriptor with reload/workers

[eltoder]

Summary

Passing a socket as stdin (--fd=0) currently does not work in modes that use subprocesses (i.e. with --reload and --workers), because config.fd is checked for truthiness instead of for None.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing eltoder:feature/one-worker (952678e) with main (6761b2c)}

[Kludex]

Thanks!