Enabling only when config is present in a project

[costa]

To enable RuboCop only when config (.rubocop.yml) is present in a project directory, I've created the global config (~/.rubocop.yml) with the following:

AllCops:
  Exclude:
    - '**/*'

But it doesn't seem to work (innocent projects with no config get rubocopped anyway).
I'm using Spacemacs (with ruby layer), and I'd like a clean solution.

Suggestions are most appreciated.

p.s. tried adding --force-exclusion to the "check command" with no effect