`XbfWriter::ProcessXamlTextBuffer` forgets to populate spObjectNodeList

### Describe the bug

In https://github.com/microsoft/microsoft-ui-xaml/blob/main/src/dxaml/xcp/core/Parser/XbfWriter.cpp#L61-L74, `XbfWriter::ProcessXamlTextBuffer` forgets to populate spObjectNodeList like it is done in https://github.com/microsoft/microsoft-ui-xaml/blob/main/src/dxaml/xcp/tools/GenXbfDLL/Main.cpp#L97-L112 . This causes a segmentation fault down the line when the (null) spObjectNodeList is accessed by https://github.com/microsoft/microsoft-ui-xaml/blob/main/src/dxaml/xcp/core/Parser/XbfWriter.cpp#L148 in order to dump the contents of the XAML document into the XBF file.

### Why is this important?

This bug makes the `XamlBinaryWriter.Write` API not work at all, causing a crash and making it impossible to compile XAML to XBF at runtime or by third party tools using the Windows App SDK.

### Steps to reproduce the bug

- Attempt to call `XamlBinaryWriter.Write` with valid input
- Observe a segmentation fault

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

1.8.260317003

### Windows version

_No response_

### Additional context

I am trying to work around https://github.com/microsoft/microsoft-ui-xaml/issues/11058 by providing XBF binary data directly instead of a XAML string, which should avoid the troubled codepath.

Inspect XbfWriter::ProcessXamlTextBuffer in src/dxaml/xcp/core/Parser/XbfWriter.cpp, especially lines 61-74 and the later access around line 148. Compare it with the corresponding logic in src/dxaml/xcp/tools/GenXbfDLL/Main.cpp lines 97-112. Reproduce XamlBinaryWriter.Write with valid XAML and confirm it no longer crashes while producing XBF data.

Written by the indexing model from the issue text.