Currently we evaluate the session parameter as truthy, but we should check whether it is None or an instance of ClientSession.

When updating the synchro tests for motor, I saw that test_helpers_with_let in test_collection.py (introduced in PYTHON-2957) was failing because the wrapped aggregate method takes only one positional argument and the test was providing an empty dict as a second argument.

If we add the validation, that test should fail and require an update to the args passed to the methods.