fix #228

[troyraen]

Fixes #228

Fallback to an empty dictionary instead of None when the alert contains no previous detections.

[troyraen]

@hernandezc1 do you want to push this change into production when you re-deploy the Cloud Functions with python 3.12?
If so:

1. I ask that you do a quick test deployment first to make sure everything works together (I see no reason why it shouldn't, but haven't tested it myself).
2. Feel free to approve and merge this PR without waiting for anything else from me.

If not, feel free to ignore it for now.

[hernandezc1]

1. I ask that you do a quick test deployment first to make sure everything works together (I see no reason why it shouldn't, but haven't tested it myself).

Successfully tested changes. No errors