(PE-455) Treat ENC classes with empty params as if they had no params

Previously, all classes specified by an ENC in parameterized format (a
hash from class to a hash of its parameters) would be evaluated as if
they had parameters. This causes a failure if the class is already
included elsewhere.

This also caused classes with empty parameters to conflict, just because
they happened to be sent as if they were parameterized. For ENCs which
always submit classes in the parameterized form, this causes strange
errors even though there isn't any real conflict, and substantially
diminishes the utility of the ENC.

With this change, ENC classes which have no parameters won't conflict
if they are already included. Classes which do have parameters continue
to conflict the same as they did before, as do classes which are
declared in Puppet code without parameters using resource syntax.