Applying dependencyProperty in the case of string type

### Describe the bug

I created the UserControl with the following DependecyProperty:

```
public string? Text
{
get => (string?)GetValue(TextProperty);
set => SetValue(TextProperty, value);
}

public static readonly DependencyProperty TextProperty =
DependencyProperty.RegisterAttached(nameof(Text), typeof(string), typeof(ColoredStringBox), new PropertyMetadata(null));
```
When the UserControl is used on one of my pages where the property Text has the value null, I would expect the same value of Text property inside UserControl, but unfortunately, the value is equal to ""(string.Empty). Is there any special reason for this or is it a bug?

### Steps to reproduce the bug

Please check the problem description!

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

None

### Windows version

_No response_

### Additional context

_No response_

The report names a UserControl and a registered DependencyProperty but provides no repository file, test, or runnable reproduction. Start by locating the dependency-property registration and related string-value handling in the WinUI codebase, then reproduce the null-to-empty behavior. Done means the behavior is explained and covered by a reliable regression test or the issue is documented as expected behavior.

Written by the indexing model from the issue text.