Fixtures get called with `NOT_USED` as a parameter when using a fixture union


```python
from pytest_cases import param_fixture, fixture_union, pytest_fixture_plus

a = param_fixture('a', ['x', 'y'])

@pytest_fixture_plus(params=[1, 2])
def b(request):
    assert request.param in [1, 2]  # this fails !
    return request.param

c = fixture_union('c', [a, b])

def test_fixture_union(c, a):
    print(c, a)
```