right_sidebar: Add functionality to render empty user list message.

In order to eliminate the usage of `::after` pseudo-element for
rendering the empty list message, this commit introduces a new function
for displaying the empty user list message on the right sidebar.

web/tests/activity.test.js

@@ -449,6 +449,28 @@ test("first/prev/next", ({override, mock_template}) => {
     assert.ok(rendered_fred);
 });
 
+test("render_empty_user_list_message", ({override, mock_template}) => {
+    const empty_list_message = "No matching users.";
+    mock_template("empty_list_widget_for_list.hbs", false, (data) => {
+        assert.equal(data.empty_list_message, empty_list_message);
+        return empty_list_message;
+    });
+
+    let appended_data;
+    override(buddy_list, "$container", {
+        append(data) {
+            appended_data = data;
+        },
+        data() {
+            return empty_list_message;
+        },
+        children: () => [],
+    });
+
+    activity_ui.render_empty_user_list_message_if_needed(buddy_list.$container);
+    assert.equal(appended_data, empty_list_message);
+});
+
 test("insert_one_user_into_empty_list", ({override, mock_template}) => {
     user_settings.user_list_style = 2;
     mock_template("presence_row.hbs", true, (data, html) => {

web/tests/user_search.test.js

@@ -9,6 +9,9 @@ const {page_params} = require("./lib/zpage_params");
 
 const fake_buddy_list = {
     scroll_container_sel: "#whatever",
+    $container: {
+        data() {},
+    },
     find_li() {},
     first_key() {},
     prev_key() {},