Fix regression in XDocumentType

[krwq]

Fixes: #44489

• partially revert https://github.com/dotnet/runtime/pull/44300/files#diff-6dcf882a73b2db05168fe9308ac11aca79bad7100caaabdefdbbedec9f3e25f0R23 (constructor argument will remain nullable but we will handle null in XNodeReader now)
• improve error messages in XLinq tests (before this it displayed number of failures and it's hard to pinpoint what actually failed, now it will show all failures)
• I have created Behavior of XDocumentType/XmlDocumentType.InternalSubset parsing is inconsistent to manual creation #44866 to open conversation what the correct behavior for internal subset for both XDocumentType and XmlDocumentType should be and summarized inconsistencies

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Details

Issue Details

Description:	Fixes regression in: #44489 partially revert https://github.com/dotnet/runtime/pull/44300/files#diff-6dcf882a73b2db05168fe9308ac11aca79bad7100caaabdefdbbedec9f3e25f0R23 (constructor argument will remain nullable but we will handle null in XNodeReader now) improve error messages in XLinq tests (before this it displayed number of failures and it's hard to pinpoint what actually failed, now it will show all failures) I have created Behavior of XDocumentType/XmlDocumentType.InternalSubset parsing is inconsistent to manual creation #44866 to open conversation what the correct behavior for internal subset for both XDocumentType and XmlDocumentType should be and summarized inconsistencies
Author:	krwq
Assignees:	stephentoub, Jozkee, buyaa-n
Labels:	area-System.Xml
Milestone:	-

[stephentoub]

/azp list

[azure-pipelines]

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• coreclr-gc-longrunning
• coreclr-gc-simulator
• runtime-coreclr outerloop
• runtime-coreclr r2r-extra
• runtime-coreclr r2r
• runtime-coreclr runincontext
• runtime-coreclr crossgen2
• runtime-libraries-coreclr outerloop
• runtime-libraries-coreclr outerloop-windows
• runtime-libraries-coreclr outerloop-linux
• runtime-libraries-coreclr outerloop-osx
• runtime
• runtime-libraries enterprise-linux
• runtime-libraries stress-http
• runtime-libraries stress-ssl
• runtime-live-build
• runtime-coreclr crossgen2 outerloop
• coreclr-release-outerloop-nightly
• sync-runtime-to-mono
• dotnet-runtime-perf
• runtime-coreclr crossgen2-composite
• runtime-jit-experimental
• runtime-coreclr libraries-jitstress
• runtime-coreclr libraries-gcstress0x3-gcstress0xc
• runtime-coreclr libraries-gcstress-extra
• dotnet-linker-tests
• runtime-coreclr ilasm
• runtime-coreclr clrinterpreter
• runtime-coreclr crossgen2-composite gcstress
• runtime-libraries-mono outerloop
• runtime-coreclr superpmi
• runtime-coreclr jitrollingbuild

[stephentoub]

/azp run runtime-libraries-coreclr outerloop

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Azure Pipelines successfully started running 1 pipeline(s).

[jeffhandley]

@krwq Please include these changes into our accumulated list of nullable annotation changes in 6.0.

[jozkee]

Source changes LGTM.

[krwq]

The failures don't seem to be related, merging