Don't fail builds for projects that are deleted when they are stuck
## What does this MR do?
Solves when dropping stuck connection.
```
Dropping stuck pending build 1545510 for runner
NoMethodError: undefined method `origin_merge_requests' for nil:NilClass
from /opt/gitlab/embedded/service/gitlab-rails/app/services/merge_requests/base_service.rb:50:in `merge_request_from'
from /opt/gitlab/embedded/service/gitlab-rails/app/services/merge_requests/base_service.rb:57:in `each_merge_request'
from /opt/gitlab/embedded/service/gitlab-rails/app/services/merge_requests/add_todo_when_build_fails_service.rb:5:in `execute'
from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/instrumentation.rb:164:in `execute'
from /opt/gitlab/embedded/service/gitlab-rails/app/models/commit_status.rb:51:in `block (2 levels) in <class:CommitStatus>'
from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/state_machines-0.4.0/lib/state_machines/eval_helpers.rb:79:in `call'
...
```
This happens, because that `default_scope` of `Projects` filters returns the projects that are not deleted, where `Ci::Build` doesn't take into account.
See merge request !4609
bf4455d146