Fix NullReferenceException in ConfigurableResponseNegotiator when Accept-Charset header is absent

[Copilot]

GetTypedHeaders().AcceptCharset returns null when no Accept-Charset header is present in the request, causing a NullReferenceException on .Count.

Change

Applied null-conditional operator in ConfigurableResponseNegotiator.GetEncoding:

// Before
if (acceptCharset.Count > 0)

// After
if (acceptCharset?.Count > 0)

When AcceptCharset is null, ?.Count yields null, the condition evaluates to false, and the method falls through to GetDefaultEncoding() as intended.

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[Copilot]

Pull request overview

Fixes a NullReferenceException in ConfigurableResponseNegotiator.GetEncoding when a request does not include an Accept-Charset header, so the negotiator falls back to the default encoding as intended.

Changes:

• Updated the Accept-Charset count check in GetEncoding to be null-aware.

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[Copilot]

acceptCharset?.Count > 0 produces a nullable boolean (because ?.Count is int?), which cannot be used directly as an if condition. This will fail to compile. Use a non-nullable condition (e.g., coalesce the count to 0, compare to true, or use a property pattern like checking acceptCharset is { Count: > 0 }).

Suggested change

	if (acceptCharset?.Count > 0)
	if (acceptCharset is { Count: > 0 })

[Copilot]

Consider adding a unit test for the reported scenario (no Accept-Charset header) to ensure GetEncoding returns the default encoding and does not throw. There are already GetEncoding_* tests in the test project, but none cover the missing-header case.