Fix root folder logic in the filesystem

[alessand10]

This PR should:
Fix #100159
Fix #99932

The previous logic appears to check that the root folder is not selected using the following bit of logic:

p_paths[0] != "res://"

But the function that populates p_path, which is _tree_get_selected(...), inserts the actively selected item at p_path[0]. This means that the root folder may still be selected, but may not be the first element in p_path:

Vector<String> FileSystemDock::_tree_get_selected(bool remove_self_inclusion, bool p_include_unselected_cursor) const {
	// Build a list of selected items with the active one at the first position.
	Vector<String> selected_strings;

	TreeItem *cursor_item = tree->get_selected();
	if (cursor_item && (p_include_unselected_cursor || cursor_item->is_selected(0)) && cursor_item != favorites_item) {
		selected_strings.push_back(cursor_item->get_metadata(0));
	}
        ... 

The root folder will only be at p_path[0] if it is the actively selected item, but otherwise it will exist at p_path[1]. To compensate for this, I have added an additional check that verifies that the root folder is not at p_path[0] and also not at p_path[1] before determining if the root folder is selected:

bool root_path_not_selected = p_paths[0] != "res://" && (p_paths.size() <= 1 || p_paths[1] != "res://");

[KoBeWi]

Looks like res:// is reliably either at p_paths[0] or p_paths[1], so this works 🤔

[akien-mga]

Thanks!