DisplaySettings: Allow unsetting the wallpaper

It was previously not possible to unset the wallpaper once set,
since loading an image from the path "" would always fail.

Applications/DisplaySettings/MonitorWidget.cpp

@@ -35,10 +35,10 @@ MonitorWidget::MonitorWidget()
 
 bool MonitorWidget::set_wallpaper(String path)
 {
-    m_desktop_wallpaper_path = path;
     auto bitmap_ptr = Gfx::Bitmap::load_from_file(path);
-    if (!bitmap_ptr)
+    if (!bitmap_ptr && !path.is_empty())
         return false;
+    m_desktop_wallpaper_path = path;
     m_desktop_wallpaper_bitmap = bitmap_ptr;
     return true;
 }