Fix digest auth dropping challenge fields with empty string values

[bysiber]

What do these changes do?

Fix DigestAuthMiddleware._authenticate silently dropping challenge fields that have empty string values.

The current code uses if value := header_pairs.get(field): which relies on truthiness check. Since empty strings are falsy in Python, any challenge field with an empty value (e.g., realm="") is excluded from self._challenge. This causes _encode() to later raise a ClientError complaining about a missing realm parameter, even though the server did provide one.

The _encode() method already acknowledges that empty realm values are valid per RFC 7616 (Section 3.3 — the realm directive SHOULD contain the host name but MAY be empty). The fix changes the check to is not None so that only genuinely absent fields are skipped while empty values are preserved.

Empty nonce values are still correctly rejected by the existing validation in _encode():

if not nonce:
    raise ClientError("Security issue: Digest auth challenge contains empty 'nonce' value")

Are there changes in behavior for the user?

Yes — digest authentication against servers that send realm="" (or other empty challenge fields) will now work instead of raising ClientError.

Is it a substantial burden for the maintainers to support this?

No, this is a single-line change.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES/ folder
  • naming format: <issue_or_PR_number>.<type>.rst
  • <type> is one of feature, bugfix, doc, removal, misc

[codspeed-hq]

Merging this PR will not alter performance

✅ 59 untouched benchmarks

---

_{Comparing bysiber:fix/digest-auth-empty-realm (46938eb) with master (e458a83)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (5efc6c8) to head (46938eb).
⚠️ Report is 382 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12097   +/-   ##
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  Coverage   98.77%   98.77%           
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  Files         128      128           
  Lines       44892    44892           
  Branches     2383     2383           
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  Hits        44342    44342           
  Misses        390      390           
  Partials      160      160           

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[bysiber]

Added a regression test for the empty realm case — it's a new parametrized entry in test_authenticate_scenarios that sends a challenge with realm="" and verifies the field is preserved in the parsed challenge dict.

[patchback]

Backport to 3.13: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.13/51d5dbac70b5321da631e224c384058948889608/pr-12097

Backported as #12107

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.14: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.14/51d5dbac70b5321da631e224c384058948889608/pr-12097

Backported as #12108

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.