XNodeReader.GetAttribute(int) should throw on invalid indices

[krwq]

Note: this class is internal but this method is accessible through XmlReader's public surface which can be created with i.e. XNode.CreateReader.

All XmlReader implementations except XNodeReader throw ArgumentOutOfRangeException on invalid indices. This specific class returns null though. Documentation of XmlReader says that ArgumentOutOfRange is expected behavior.

Also same method returns null when reader state is not in interactive mode - likely this should be replaced with InvalidOperationException.

Found with nullable annotations.

[ghost]

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[krwq]

I think we should fix this for consistency but there is no urgency here considering this issue has been there for years and no one has complained.