Fix error when swapping a ref with a trivial indexing transform.
Without this fix, the added test case fails with:
```
...
jax/_src/state/discharge.py:416: in _swap_discharge_rule
z, x_new = _swap_discharge(x, val, idx, tree)
jax/_src/state/discharge.py:421: in _swap_discharge
return transform_swap_array(x, transforms, val)
jax/_src/state/discharge.py:396: in transform_swap_array
result_val = lax_slicing.dynamic_update_slice(
jax/_src/lax/slicing.py:215: in dynamic_update_slice
start_indices = _dynamic_slice_indices(operand, start_indices)
...
AttributeError: 'NoneType' object has no attribute 'ndim'
```
from encountering a None when computing the `result_val`.
