_tmp_session should validate session input

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.