Specify an association dependent behavior
Specify an association dependent behavior

`:nullify` is the default behavior, but let's specify it to address
the rubocop warning.

See also
https://docs.mongodb.com/mongoid/master/tutorials/mongoid-relations/#dependent-behaviour

tomatoes-app/tomatoes,tomatoes-app/tomatoes,potomak/tomatoes,tomatoes-app/tomatoes,potomak/tomatoes,tomatoes-app/tomatoes,potomak/tomatoes