Merge pull request #5801 from AvaloniaUI/fixes/osx-appmenu

[osx] if app only has an app menu and no window menu, ensure its appl…

native/Avalonia.Native/src/OSX/window.mm

@@ -1936,6 +1936,10 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
         
         [NSApp setMenu:_menu];
     }
+    else
+    {
+        [self showAppMenuOnly];
+    }
 }
 
 -(void) showAppMenuOnly