Don't validate when there's nothing to validate

This would otherwise mean that the consumers would have
to prepare for e.g. validation of empty list, which can
already be done at this level

src/eon_type.erl

@@ -281,6 +281,8 @@ typecheck(#spec{term=Term0, type=Type, p_have=P_have}) ->
     eon_type_rec ->
       Term = ?unlift(check_obj(Term0, Type:decl(Term0, P_have))),
       case erlang:function_exported(Type, extra_validation, 2) of
+        true when Term =:= [] ->
+          Term;
         true ->
           case Type:extra_validation(Term, P_have) of
             ok -> Term;