feat: prepare CSS to hide empty battery group (Waybar PR #4941)

defaults/xdg/waybar/style.css

@@ -114,6 +114,19 @@ widget widget>* {
     color: #cc3436;
 }
 
+/* --- Battery group (hidden when empty, requires Waybar PR #4941) --- */
+
+#bat.empty,
+#bat.empty * {
+    min-width: 0;
+    min-height: 0;
+    padding: 0;
+    margin: 0;
+    border: none;
+    font-size: 0;
+    opacity: 0;
+}
+
 /* --- Battery --- */
 
 #battery {