Fix Count and ReferenceManyCount should allow to override the default sort

[slax57]

Fixes #8731

[slax57]

In the end, I'm not so sure about setting the sort prop to null, because dataProvider implementations might contain code expecting non-null sort props, like:

const { field, order } = params.sort;

I think it's safer to keep the default value, and simply add the ability for the developers to override it.