stream_topic_link: Add support for empty string topic in syntax.

This commit adds support for empty string as a valid topic name
in syntax for linking to topics.

The server stores it after empty string is replaced with
`realm_empty_topic_display_name` and wrapped with an <em> tag.

The web client parses the rendered_content and updates
the topic_name part in the HTML with topic_name in user's language
+ wraps it in a <span> tag with 'empty-topic-display' css class.

zerver/lib/markdown/__init__.py

@@ -177,7 +177,7 @@ STREAM_TOPIC_LINK_REGEX = rf"""
                      \#\*\*                          # and after hash sign followed by double asterisks
                          (?P<stream_name>[^\*>]+)    # stream name can contain anything except >
                          >                           # > acts as separator
-                         (?P<topic_name>[^\*]+)      # topic name can contain anything
+                         (?P<topic_name>[^\*]*)      # topic name can be an empty string or contain anything
                      \*\*                            # ends by double asterisks
                    """
 
@@ -2066,8 +2066,16 @@ class StreamTopicPattern(StreamTopicMessageProcessor):
         topic_url = hash_util_encode(topic_name)
         link = f"/#narrow/channel/{stream_url}/topic/{topic_url}"
         el.set("href", link)
-        text = f"#{stream_name} > {topic_name}"
-        el.text = markdown.util.AtomicString(text)
+
+        if topic_name == "":
+            topic_el = Element("em")
+            topic_el.text = Message.EMPTY_TOPIC_FALLBACK_NAME
+            el.text = markdown.util.AtomicString(f"#{stream_name} > ")
+            el.append(topic_el)
+        else:
+            text = f"#{stream_name} > {topic_name}"
+            el.text = markdown.util.AtomicString(text)
+
         return el, m.start(), m.end()