Use None check for request.id to preserve falsy IDs

[bysiber]

The request.id property uses if not self._id to decide whether to generate/fetch the ID. This treats any falsy value (including 0, "") as "not yet set," causing the ID to be regenerated on every property access.

A custom generate_id() returning 0 or an integer request ID header value of "0" (which gets cast to int(0)) would trigger re-generation every time request.id is read. Switching to is None preserves any explicitly-set falsy ID while still triggering lazy initialization when genuinely unset.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.817%. Comparing base (785d77f) to head (b712c5c).

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3143       +/-   ##
=============================================
+ Coverage   87.793%   87.817%   +0.024%     
=============================================
  Files          105       105               
  Lines         8143      8143               
  Branches      1290      1290               
=============================================
+ Hits          7149      7151        +2     
+ Misses         687       686        -1     
+ Partials       307       306        -1     

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