Fix for the issue in registration migration,
when template might not exist while migrating the db.

See https://github.com/theforeman/foreman/pull/8256#discussion_r824099694

Error:

foreman-rake db:migrate --trace --verbose
'ErbParser' is ignored.
'RubyParser' is ignored.
** Invoke db:migrate (first_time)
** Invoke db:load_config (first_time)
** Invoke environment (first_time)
** Execute environment
API controllers newer than Apipie cache! Run apipie:cache rake task to regenerate cache.
** Execute db:load_config
** Invoke plugin:refresh_migrations (first_time)
** Invoke environment
** Execute plugin:refresh_migrations
** Execute db:migrate
== 20210115124508 TemplateKindRegistration: migrating =========================
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

undefined method operatingsystems' for nil:NilClass 
/usr/share/foreman/db/migrate/20210115124508_template_kind_registration.rb:26:in block in up'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/relation/delegation.rb:87:in each' /opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/relation/delegation.rb:87:in each'
/usr/share/foreman/db/migrate/20210115124508_template_kind_registration.rb:25:in up' /opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:831:in exec_migration'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:812:in block (2 levels) in migrate' /opt/rh/rh-ruby27/root/usr/share/ruby/benchmark.rb:293:in measure'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:811:in block in migrate' /opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/connection_adapters/abstract/connection_pool.rb:471:in with_connection'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:810:in migrate' /opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:1002:in migrate'
/opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:1310:in block in execute_migration_in_transaction' /opt/theforeman/tfm/root/usr/share/gems/gems/activerecord-6.0.3.7/lib/active_record/migration.rb:1361:in block in ddl_transaction'

/opt/rh/rh-ruby27/root/usr/bin/rake:23:in `<main>'

Applied in changeset foreman|73101ef97e7fbb1b34a04b00e2a2d005b97e27d7.