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`.

Author: jburnim

jax/_src/state/discharge.py

@@ -364,7 +364,7 @@ def transform_swap_array(x, transforms, val):
       case indexing.NDIndexer():
         indexer = transform
         if _is_trivial_indexer(indexer):
-          _results.append(None)
+          _results.append(_results[-1])
           continue
         # If everything in the indexer is a slice or ()-shaped, we can also
         # use `lax.dynamic_slice` with 1-sized slices for ()-shaped indices.

tests/state_test.py

@@ -639,6 +639,26 @@ def f(a_ref):
     refval, = core.eval_jaxpr(discharged_jaxpr, discharged_consts, inval)
     self.assertTrue((refval == inval.at[jnp.array([0, 1])].set(1.)).all())
 
+  def test_discharge_swap(self):
+    def f(a_ref):
+      a = ref_swap(
+          a_ref.at[0:4, 0:3, 0:2].at[1:3, :, 0],
+          (slice(None), slice(1, 3)),
+          jnp.zeros((2, 2), jnp.float32))
+      return [a + 1]
+    in_avals = [shaped_array_ref((4, 3, 2), jnp.float32)]
+    stateful_jaxpr, _, consts, () = pe.trace_to_jaxpr_dynamic(
+        lu.wrap_init(f), in_avals)
+
+    discharged_jaxpr, () = discharge_state(stateful_jaxpr, consts)
+    self.assertLen(discharged_jaxpr.invars, 1)
+    self.assertLen(discharged_jaxpr.outvars, 2)
+
+    inval = jnp.arange(24., dtype=jnp.float32).reshape((4, 3, 2))
+    outval, refval = core.eval_jaxpr(discharged_jaxpr, (), inval)
+    self.assertArraysEqual(outval, inval[1:3, 1:3, 0] + 1)
+    self.assertArraysEqual(refval, inval.at[1:3, 1:3, 0].set(0))
+
   def test_discharge_addupdate(self):
     def f(a_ref, b):
       ref_addupdate(a_ref, (), b + 1)