Validate data and raise exception

[verovaleros]

Description

This PR fixes a bug in which the Alpha and Alpha7 models will report success even when there's no data available to run the models.

• The Alpha model was modified to validate that the attack cache is not empty before attempting to build the blocklist.
• An exception is now raised if the attack cache is empty so the run_model will report the exception instead of success.

Fixes #66

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tested locally with many start and end date combinations, including the example reported in Alpha(s) models report 'success' when no data is available #66

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

Changes to the documentation

• I have made corresponding changes to the documentation