NoMethodError: Repositories::LfsLocksApiController#verify - undefined method `id' for nil:NilClass
### Problem
Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1299955
```
NoMethodError: undefined method `id' for nil:NilClass (NoMethodError)
groups = locks.partition { |lock| lock.user_id == user.id }
^^^
from app/controllers/repositories/lfs_locks_api_controller.rb:61:in `block in split_by_owner'
from active_record/relation/delegation.rb:88:in `each'
from active_record/relation/delegation.rb:88:in `each'
from app/controllers/repositories/lfs_locks_api_controller.rb:61:in `partition'
from app/controllers/repositories/lfs_locks_api_controller.rb:61:in `split_by_owner'
from app/controllers/repositories/lfs_locks_api_controller.rb:32:in `verify'
```
### Proposal
* Try to reproduce this error
* Handle the case when the user is missing
* Add a test for this case
issue