Avoid auto-vivification for runner token

This fixes an issue with the runner token not being populated correctly if token were omitted from a Chef role.

Previously if token were not specified, the AttributesWriter would quietly assign the value to {} because of Chef's auto-vivify feature.

This means that for runner tokens secrets to work, a role must specify a null value of token.

However, for Chef 16, the behavior of nil values has changed: https://docs.chef.io/release_notes_client/#15.0.293-node-attributes-deep-merge-nil-values. This means that we need to omit token from the role for the attributes to be resolved correctly.