Add nullability annotations to System.Private.Xml.Linq project

[jozkee]

Contributes to #2339

[ghost]

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[krwq]

should all resolvers above also be nullable?

[jozkee]

Yes, and also in other places where a IXmlNamespaceResolver is taken, not sure if everywhere but we should be able to take null in other cases, e.g:

runtime/src/libraries/System.Private.Xml/src/System/Xml/Core/XmlReader.cs

Line 381 in f0461cb

publicvirtualobjectReadContentAs(TypereturnType,IXmlNamespaceResolvernamespaceResolver)

The docs also mention that the param can be null https://docs.microsoft.com/dotnet/api/system.xml.xmlreader.readcontentas?view=netcore-3.1

[jeffhandley]

Do the other places IXmlNamespaceResolver is taken still need to be updated to be nulalble?

[jozkee]

XmlReader.ReadContentAs(Type, IXmlNamespaceResolver) is yet as not-nullable so I assume is the same for the other cases if they exists.

[jozkee]

I did a scan through the methods that can take IXmlNamespaceResolver and found a few ones that could be changed to IXmlNamespaceResolver?. I submitted #41079 with the changes.

[krwq]

LGTM, thanks for fixing, please ensure build errors are fixed or known issues

[jozkee]

CI issues are unrelated, see #41006.