Include even empty attributes in as_json() output

The JSON parser frowns on nil attributes on input. Making rOCCI's output re-parsable.
Spec changes to complement as_json() change

Complementing commit d5f3e130a5b5c9e2d4801a8ff3f24dd6cc966fb2
Do not pass attributes explicitly on calling init

The right default will be used anyway and the original input has a wrong type (does not respond to :convert)
Raise exception if directory does not exist
Specs for #register_files()