Use `:empty_project` where possible in service specs

spec/services/projects/update_service_spec.rb

spec/services/projects/upload_service_spec.rb

spec/services/search_service_spec.rb

spec/services/slash_commands/interpret_service_spec.rb

spec/services/spam_service_spec.rb

spec/services/system_hooks_service_spec.rb

spec/services/system_note_service_spec.rb

spec/services/tags/create_service_spec.rb

spec/services/tags/destroy_service_spec.rb

spec/services/test_hook_service_spec.rb

spec/services/todo_service_spec.rb

spec/services/update_release_service_spec.rb

spec/services/users/destroy_spec.rb

...ervices/users/refresh_authorized_projects_service_spec.rb

...issuable_create_service_slash_commands_shared_examples.rb