Git::Annotations with null `git_version_id`

[fbacall]

Possibly introduced with Workflow RO-Crate submission API - there are persisted git annotations with a null value for git_version_id.

This was detected because the uniqueness validation was failing:

validates :key, uniqueness: { scope: [:git_version_id, :path, :value] }

as it uses the nil git_version_id as part of the scope