Use an empty string rather than "broken"

If we fail to retrieve the title, then there is no title,
though this doesn't mean the same as "no window",
for which we have NULL.

wdmtg.c

@@ -303,7 +303,7 @@ x_window_title(xcb_window_t window)
 	GString *title;
 	if (!(title = x_text_property(window, gen.atom_net_wm_name))
 	 && !(title = x_text_property(window, XCB_ATOM_WM_NAME)))
-		return g_strdup("broken");
+		return g_strdup("");
 	return g_string_free(title, false);
 }