Hide the popover title area if it is empty.

This makes our popovers-as-menus look a bit better.

(imported from commit 53b8be0a4f16fc7a7884b3fcb276a9ab85fe6f24)

zephyr/static/styles/zephyr.css

@@ -1257,6 +1257,10 @@ table.floating_recipient {
     line-height: inherit;
 }
 
+.popover-title:empty {
+    display: none;
+}
+
 .popover_info {
     text-align: center;
 }