Check dlm.E001 behaviour for detecting presence of migrations does not match create-max-migration-files behaviour

[ahumeau]

Hi Adam,

First thank you for this package, it solves a problem that I have been having for some time. I am excited to try it out.

The inconsistency was introduced by 0f3741c .

create-max-migration-files checks if len(migration_details.names) > 0, the check does not.

I am not opening a PR because I want to know your opinion on wether or not it would be a better solution to move that check in MigrationDetails.has_migrations instead of just adding it in check_max_migration_files.

[adamchainz]

I feel like I had a reason at the time why I didn't push that check into has_migrations, but I can't remember why. Let's do it and see what breaks.