Fix XmlElement deserialization for empty elements

[Copilot]

Fixed an issue where empty XML elements were not properly deserialized when using XmlElement properties, causing subsequent elements to be incorrectly consumed or skipped.

Problem

When deserializing XML with empty elements followed by other elements, the XmlElement property would return null for empty elements, potentially causing misalignment in the parsing of subsequent elements. For example:

public class Root
{
    public XmlElement Description { get; set; }
    public string Name { get; set; }
}

var serializer = new XmlSerializer(typeof(Root));

// This would incorrectly return Description = null instead of an empty XmlElement
var result = serializer.Deserialize(new StringReader(@"<root><description></description><name>Test</name></root>"));

Root Cause

The ReadXmlNode method in XmlSerializationReader was returning null for empty XML elements instead of creating an appropriate empty XmlElement node. This occurred for both self-closing elements (<description/>) and empty elements with explicit tags (<description></description>).

Solution

Modified the ReadXmlNode method to:

1. Detect when an element is empty (either self-closing or with no content between tags)
2. Create an empty XmlElement with the correct local name and namespace
3. Preserve any attributes that were present on the empty element
4. Ensure consistent behavior between self-closing and explicit empty tag formats

Tests Added

Added comprehensive tests to verify:

• Empty elements create proper XmlElement objects instead of null
• Self-closing and explicit empty elements behave identically
• Subsequent elements are correctly parsed after empty elements
• Non-empty elements continue to work as expected

Fixes #80699.

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