Projects::TreeController#create_dir: no implicit conversion of nil into String

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/953134

TypeError: no implicit conversion of nil into String
  from app/controllers/projects/tree_controller.rb:66:in `join'
  from app/controllers/projects/tree_controller.rb:66:in `assign_dir_vars'

Problem

Missing validation for branch_name and dir_name presence leads to 500 error.