How can I define a paragraph style that does not assert any character attributes.
No way. But what for?
Under exceptional circumstances, it is possible to define paragraph styles which take their font face and size attributes from outside the style machinery. But, in the end, even such styles will end up with specific (predictable) font face face and size. Tell us what you want to do (by editing tour question, not using an answer) and we’ll evaluate if this not-recommended-way-for-newbies fits your goal.