Describe the bug

Most GetXYZ(int) (e. g. GetInt32, GetString, etc) methods on SqlDataReader will throw if they encounter a DBNull.Value and cannot return it. The exception is GetValue which can just return DBNull.Value directly.

GetXmlReader is another exception but with (in my opinion) less reasonable behavior. When called on a column whose value is SQL NULL, it returns a non-null XmlReader object which fails when read from.

To reproduce

Include a complete code listing (or project/solution) that we can run to reproduce the issue.

var connectionString = "...";
using var conn = new Microsoft.Data.SqlClient.SqlConnection(connectionString);
conn.Open();
using var cmd = conn.CreateCommand();
cmd.CommandText = "select cast(null as xml)";
var reader = cmd.ExecuteReader();
reader.Read();

Console.WriteLine(reader.IsDBNull(0)); // true
using var xmlReader = reader.GetXmlReader(0); // works, does not return null
Console.WriteLine(XDocument.Load(xmlReader)); // throws InvalidOperationException "The XmlReader state should be interactive"
//Console.WriteLine(reader.GetString(0)); // throws SqlNullValueException "Data is Null. This method or property cannot be called on Null values."

Expected behavior

For consistency with the other get methods, I think GetXmlReader should throw SqlNullValueException when called on a DBNull column.

This behavior would be useful when processing query results because any errors due to expected non-null values fail fast early rather than manifesting later as a different-seeming exception.

Further technical details

Microsoft.Data.SqlClient version: 4.0.0
.NET target: .NET 5
SQL Server version: SQL Server 2017
Operating system: Windows 10