Initialize wxFileDialog::m_data in default ctor in wxMSW too

This corrects a problem introduced in 11ed91af8f (Add missing default
constructor of wxFileDialog in wxMSW, 2023-12-03) as m_data remained
uninitialized when default ctor was used.

See #24113.

include/wx/msw/filedlg.h

@@ -79,7 +79,7 @@ private:
 
     // Extra data, possibly null if not needed, use MSWData() to access it if
     // it should be created on demand.
-    wxFileDialogMSWData* m_data;
+    wxFileDialogMSWData* m_data = nullptr;
 
 
     wxDECLARE_DYNAMIC_CLASS(wxFileDialog);