Add null checks in various XML methods

[krwq]

Fixes https://github.com/dotnet/runtime/issues/41061

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Issue Details

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Fixes https://github.com/dotnet/runtime/issues/41061

Author:	krwq
Assignees:	-
Labels:	area-System.Xml
Milestone:	-

[krwq]

no changes, just rebase

[krwq]

Seems there was a test failure where we for some reason expected InvalidOperationException for null argument (with completely useless error message and NullRef on the inner exception), see here for more details: https://github.com/dotnet/runtime/pull/73243#discussion_r939709955 - I went ahead and removed this (since I already added test asserting ArgumentNullException) - I don't expect any particular impact from this change since it only happens for null arguments which suggests design error rather than something which app could potentially catch - makes more sense to throw more useful error message. Also now we match the behavior written in docs

[danmoseley]

Any of these worth a breaking change note? (I'm guessing not)

[krwq]

I don't think so, we're now matching behavior to already documented behavior and this exception seems very unlikely to be found by random input - most likely will be only seen during app-design time by developer.