Types: Fix type annotation for default_encoding parameter

[JelleZijlstra]

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The default encoding function is allowed to return None, in which case the encoding property of httpx.Response defaults to utf-8.

The example in https://www.python-httpx.org/advanced/#using-character-set-auto-detection that passes a default_encoding function uses a function that may return None.

Checklist

• [x] 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.