[Group 5] Enable nullable annotations for Microsoft.Extensions.Configuration.Xml

[maxkoshevoi]

Related to #43605

System.Security.Cryptography.Xml is not annotated yet (maybe will get around to it in some time), but it's only used in one place where there's no nullability questions (#66892 (comment)).

Questions:

• CreateNavigator() can return null here
  

  runtime/src/libraries/Microsoft.Extensions.Configuration.Xml/src/XmlDocumentDecryptor.cs

  Line 101 in 0ceb116

  return document.CreateNavigator().ReadSubtree();

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Issue Details

---

Related to #43605

System.Security.Cryptography.Xml is not annotated yet (maybe will get around to it in some time), but it's only used in one place where there's no nullability questions.

Questions:

• CreateNavigator() can return null here
  

  runtime/src/libraries/Microsoft.Extensions.Configuration.Xml/src/XmlDocumentDecryptor.cs

  Line 101 in 0ceb116

  return document.CreateNavigator().ReadSubtree();

Author:	maxkoshevoi
Assignees:	-
Labels:	new-api-needs-documentation, area-Extensions-Configuration, community-contribution
Milestone:	-

[eerhardt]

CreateNavigator() can return null here

Looking at the code, I believe this can only return null when the XmlNode you call it on is one of these:

runtime/src/libraries/System.Private.Xml/src/System/Xml/Dom/XmlDocument.cs

Lines 784 to 789 in c3843c5

	case XmlNodeType.EntityReference:
	case XmlNodeType.Entity:
	case XmlNodeType.DocumentType:
	case XmlNodeType.Notation:
	case XmlNodeType.XmlDeclaration:
	return null;

Since we are calling it on XmlDocument, I don't believe this will ever return null, so the suppression is safe.

@krwq - is this something we can fix in the annotations for System.Xml? I think we can remove the ? here:

runtime/src/libraries/System.Private.Xml/src/System/Xml/Dom/XmlDocument.cs

Lines 771 to 774 in c3843c5

	public override XPathNavigator? CreateNavigator()
	{
	return CreateNavigator(this);
	}

XmlDocument.CreateNavigator() should never return null.

[eerhardt]

LGTM. Thanks again @maxkoshevoi.