Consider making `TokenExtensions.UpdateTokenValue()`'s `tokenValue` parameter nullable

[kevinchalet]

Summary

Likely due to an oversight, the tokenValue parameter exposed by the TokenExtensions.UpdateTokenValue() API isn't currently declared as a string?, even though this parameter is expected to be nullable (it's not null-guarded and it updates a dictionary entry for which values are deliberately nullable).

Motivation and goals

Fixing the signature would offer a better indication that the tokenValue parameter is actually nullable.

Risks / unknowns

This change only affects compilation (since nullable references are hints) and has no risk of runtime regression.

Examples

// Currently logs a CS8604 warning as `GetTokenValue()` returns a `string?`.
properties.UpdateTokenValue("refresh_token", result.Properties.GetTokenValue("refresh_token"));

[dotnet-policy-service]

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