Based on the problem statement, I understand the issue clearly. The bug is in the Django Messages framework where `MessageEncoder` checks for truthiness of `extra_tags` instead of checking if it's not `None`. This causes empty strings to be treated as falsy and not serialized, resulting in `None` when deserialized.

Let me create the patch based on the standard Django codebase structure:

```
--- a/django/contrib/messages/storage/cookie.py
+++ b/django/contrib/messages/storage/cookie.py
@@ -14,7 +14,7 @@ class MessageEncoder(json.JSONEncoder):
     def default(self, obj):
         if isinstance(obj, Message):
             # Using 0/1 here instead of False/True to produce more compact json
-            is_safedata = 1 if isinstance(obj.message, SafeData) else 0
-            message = [self.message_key, is_safedata, obj.level, obj.message]
-            if obj.extra_tags:
+            is_safedata = 1 if isinstance(obj.message, SafeData) else 0
+            message = [self.message_key, is_safedata, obj.level, obj.message]
+            if obj.extra_tags is not None:
                 message.append(obj.extra_tags)
             return message
         return super().default(obj)
```

Wait, let me provide a cleaner patch. The issue is that the encoder checks `if obj.extra_tags:` which is falsy for empty string, so it doesn't include it. The fix should check `if obj.extra_tags is not None:` instead.

--- a/django/contrib/messages/storage/cookie.py
+++ b/django/contrib/messages/storage/cookie.py
@@ -17,7 +17,7 @@ class MessageEncoder(json.JSONEncoder):
         if isinstance(obj, Message):
             # Using 0/1 here instead of False/True to produce more compact json
             is_safedata = 1 if isinstance(obj.message, SafeData) else 0
             message = [self.message_key, is_safedata, obj.level, obj.message]
-            if obj.extra_tags:
+            if obj.extra_tags is not None:
                 message.append(obj.extra_tags)
             return message
         return super().default(obj)