Allow set_disease_types to continue from where it left off

If the command fails or is killed for whatever reason, we should avoid
it processing vaccination records that it has already done.

Jira-Issue: MAV-2858

Author: thomasleese

lib/tasks/data_migration.rake

@@ -10,6 +10,7 @@ namespace :data_migration do
   task set_disease_types: :environment do
     VaccinationRecord
       .includes(:vaccine)
+      .where(disease_types: nil)
       .find_each do |vaccination_record|
         disease_types =
           vaccination_record.vaccine&.disease_types ||