Help: Make sure the home page is actually shown when opening the app

Previously `open_page` was called multiple times when opening the
application, once for the actual page, but also in
`tree_view.on_selection_change`. At startup there is no valid selection
yet, which caused an empty page to be loaded. Prevent this by early
returning if the `path` is null.

Userland/Applications/Help/main.cpp

@@ -163,6 +163,9 @@ int main(int argc, char* argv[])
 
     tree_view.on_selection_change = [&] {
         String path = model->page_path(tree_view.selection().first());
+        if (path.is_null())
+            return;
+
         history.push(path);
         update_actions();
         open_page(path);