NavigationView. AppBar not showed when NavigationPane is not provided🐛

[AndriiHarashchak]

Describe the bug
I am building a windows application with a custom title bar. I have a few pages that don't need a navigation pane, so I am trying to use NavigationView with only "appBar" and "content" widgets provided. In this case, "appBar " is not shown, and "content" fills all window space. In this case I can`t do any manipulations with app window (drag, close, minimize, etc.) When I add "pane" everything looks good.

To Reproduce
Steps to reproduce the behavior:

1. Create a page-widget (in my case Stateful widget) with NavigationView which includes only "appBar" and "content"
2. Run windows application with this page.

Expected behavior
AppBar is shown even "pane" is not provided.