TrackingTest no longer assigns None to variables (keras-team#21106)

JAX will soon fail when `jnp.array` is called with None, so this test will be broken under newer JAX versions if kept as is.

Author: superbobry

keras/src/utils/tracking_test.py

@@ -20,7 +20,7 @@ def test_untracking_in_tracked_list(self):
         v2 = backend.Variable(2.0)
         lst = tracking.TrackedList([], tracker)
         lst.append(v1)
-        lst.append(None)
+        lst.append(float("nan"))
         lst.append(v2)
         lst.append(0)
 
@@ -38,7 +38,7 @@ def test_untracking_in_tracked_list(self):
 
         lst2 = tracking.TrackedList([], tracker)
         lst2.append(v1)
-        lst2.append(None)
+        lst2.append(float("nan"))
         lst2.append(v2)
         lst2.append(0)