topic_link_util: Add support for empty topics in the frontend.

Though the frontend module generated correct urls for
empty topics, it would sometime generate incorrect
link text in the fallback md link syntax. (eg, if
empty string was provided as the topic name).

This commit fixes that.

web/src/topic_link_util.ts

@@ -4,6 +4,7 @@ import assert from "minimalistic-assert";
 
 import * as hash_util from "./hash_util.ts";
 import * as stream_data from "./stream_data.ts";
+import * as util from "./util.ts";
 
 const invalid_stream_topic_regex = /[`>*&[\]]|(\$\$)/g;
 
@@ -64,14 +65,15 @@ export function get_topic_link_content(
     const escape = html_escape_markdown_syntax_characters;
     if (topic_name !== undefined) {
         const stream_topic_url = hash_util.by_stream_topic_url(stream_id, topic_name);
+        const topic_display_name = util.get_final_topic_display_name(topic_name);
         if (message_id !== undefined) {
             return {
-                text: `#${escape(stream_name)} > ${escape(topic_name)} @ 💬`,
+                text: `#${escape(stream_name)} > ${escape(topic_display_name)} @ 💬`,
                 url: `${stream_topic_url}/near/${message_id}`,
             };
         }
         return {
-            text: `#${escape(stream_name)} > ${escape(topic_name)}`,
+            text: `#${escape(stream_name)} > ${escape(topic_display_name)}`,
             url: stream_topic_url,
         };
     }

web/tests/topic_link_util.test.cjs

@@ -13,6 +13,9 @@ mock_esm("../src/user_settings", {
         web_channel_default_view: settings_config.web_channel_default_view_values.channel_feed.code,
     },
 });
+mock_esm("../src/state_data", {
+    realm: {realm_empty_topic_display_name: "general chat"},
+});
 
 const sweden_stream = {
     name: "Sweden",
@@ -125,6 +128,23 @@ run_test("stream_topic_link_syntax_test", () => {
         "[#Sweden > &a[b](#narrow/channel/1-Sweden/topic/.26a.5Bb)",
     );
 
+    assert.equal(topic_link_util.get_stream_topic_link_syntax("Sweden", ""), "#**Sweden>**");
+
+    assert.equal(
+        topic_link_util.get_stream_topic_link_syntax("$$MONEY$$", ""),
+        "[#$$MONEY$$ > translated: general chat](#narrow/channel/6-.24.24MONEY.24.24/topic/)",
+    );
+
+    assert.equal(
+        topic_link_util.get_fallback_markdown_link("Sweden", "abc", 123),
+        "[#Sweden > abc @ 💬](#narrow/channel/1-Sweden/topic/abc/near/123)",
+    );
+
+    assert.equal(
+        topic_link_util.get_fallback_markdown_link("Sweden", "", 123),
+        "[#Sweden > translated: general chat @ 💬](#narrow/channel/1-Sweden/topic//near/123)",
+    );
+
     // Only for full coverage of the module.
     assert.equal(topic_link_util.escape_invalid_stream_topic_characters("Sweden"), "Sweden");
     assert.equal(