Node slice indexing with stop index 0 is broken

[mark-koch]

>>> from hugr import Node
>>> n = Node(0, _num_out_ports=1)
>>> list(n[:0])

Expected output is [], but it actually gives [OutPort(Node(0), 0)].

This is because stop index 0 is evaluated as falsy in

hugr/hugr-py/src/hugr/node_port.py

Line 173 in 95b3949

stop = index.stop or self._num_out_ports

We should replace this with an explicit check for None.