TabView throws ArgumentException when TabViewItem has no effective header

### Describe the bug

TabView throws an unhandled ArgumentException (E_INVALIDARG) when a TabViewItem has no effective header value while containing child content.

The exception occurs when TabViewItem.Header is not set at all, or when it is bound to an empty string. Binding a non-empty string avoids the issue.

This exception originates from WinUI/WinRT and is surfaced via Microsoft.UI.Xaml.UnhandledException.

### Steps to reproduce the bug

1. Create a new WinUI 3 (Desktop) project
2. Add a TabView using TabItemsSource and TabItemTemplate
3. Define a TabViewItem that contains child content
4. Do not set TabViewItem.Header, or bind it to an empty string
5. Run the application
6. An unhandled ArgumentException is raised

XAML
```XAML

```

Code Behind
```csharp
public ObservableCollection Items { get; } = [""];
```

### Expected behavior

TabView should handle TabViewItem instances with no header value gracefully, or reject them with a clear and documented error.

An empty or unset header should not cause an unhandled exception.

### Screenshots

_No response_

### NuGet package version

Windows App SDK 1.8.3: 1.8.251106002

### Packaging type

Unpackaged, Packaged (MSIX)

### Windows version

Windows 11 version 24H2 (26100, June 2025 Update)

### IDE

Other

### Additional context

IDE:
Microsoft Visual Studio Professional
December 2025 Feature Update
Channel: Stable
Version: 18.1.0

Exception details:

- Exception type: System.ArgumentException
- HRESULT: 0x80070057 (E_INVALIDARG)
- Message:
"The parameter is incorrect.

The parameter is incorrect."
- ParamName: null
- InnerException: null
- StackTrace: null

Additional observations:
- The exception occurs even if TabViewItem.Header is not bound at all.
- Binding an empty string ("") produces the same result.
- Binding a non-empty string (e.g. "a") avoids the exception.
- If TabViewItem contains no child content, the exception does not occur, even when the header is missing or empty.

Workaround:
Ensure that TabViewItem.Header always has a non-empty value.

Start by reproducing the supplied WinUI 3 Desktop XAML example with an empty Items value and child content in TabViewItem. Investigate the TabView and TabViewItem handling involved in that reproduction; done means missing or empty headers no longer produce an unhandled ArgumentException while non-empty headers continue to work.

Written by the indexing model from the issue text.