Error when constructing drop message

[Zlopez]

I found this issue today on staging instance of the-new-hotness. It looks like we are hitting some corner case in hotness_schema.

[2021-06-03 18:48:18,605 hotness.hotness_consumer INFO] Handling anitya msg 'a69a0bdf-75ba-43ab-8bba-43b091a87fc7'
[2021-06-03 18:48:18,605 hotness.hotness_consumer INFO] No 'Fedora' mapping for 'dart'. Dropping.
[2021-06-03 18:48:18,605 hotness.notifiers.fedora_messaging INFO] publishing topic 'hotness.update.drop'
[2021-06-03 18:48:18,606 hotness.use_cases.notify_user_use_case ERROR] Notify user use case failure
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/hotness/notifiers/fedora_messaging.py", line 93, in notify
    msg = message_class(topic=topic, body=body)
  File "/usr/lib/python3.8/site-packages/fedora_messaging/message.py", line 318, in __init__
    self._properties = properties or self._build_properties(headers)
  File "/usr/lib/python3.8/site-packages/fedora_messaging/message.py", line 328, in _build_properties
    headers.update(self._filter_headers())
  File "/usr/lib/python3.8/site-packages/fedora_messaging/message.py", line 348, in _filter_headers
    for package in self.packages:
  File "/usr/local/lib/python3.8/site-packages/hotness_schema/messages.py", line 106, in packages
    set(
KeyError: 'pop from an empty set'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/hotness/use_cases/notify_user_use_case.py", line 56, in notify
    result = self.notifier.notify(
  File "/usr/local/lib/python3.8/site-packages/hotness/notifiers/fedora_messaging.py", line 102, in notify
    output["msg_id"] = msg.id
UnboundLocalError: local variable 'msg' referenced before assignment