Use '[deleted]' instead of failing if you edit a message to empty.

Trac #1571

(imported from commit a51602d72aa2b89ce451946ae7f5a5363a6421f4)

zerver/lib/actions.py

@@ -990,8 +990,8 @@ def do_update_message(user_profile, message_id, subject, content):
                 first_rendered_content = old_edit_history_event['prev_rendered_content']
 
     if content is not None:
-        if content == "":
-            raise JsonableError("Message can't be empty")
+        if len(content.strip()) == 0:
+            content = "[deleted]"
         if len(content) > MAX_MESSAGE_LENGTH:
             raise JsonableError("Message too long")
         rendered_content = message.render_markdown(content)