Enable RaggedTensor dispatch for tf.where with x=y=None.

PiperOrigin-RevId: 225408570

tensorflow/python/ops/ragged/ragged_dispatch.py

@@ -76,6 +76,8 @@ def _get_arg_infos(func, arg_names):
 
 def _is_convertible_to_tensor(value):
   """Returns true if `value` is convertible to a `Tensor`."""
+  if value is None:
+    return True
   if isinstance(value,
                 (ops.Tensor, variables.Variable, np.ndarray, int, float, str)):
     return True

tensorflow/python/ops/ragged/ragged_dispatch_test.py

@@ -555,6 +555,10 @@ class RaggedElementwiseOpsTest(ragged_test_util.RaggedTensorTestCase,
                 ragged_factory_ops.constant_value([[b'A', b'B'], [b'C']]),
                 ragged_factory_ops.constant_value([[b'a', b'b'], [b'c']])),
           expected=ragged_factory_ops.constant_value([[b'A', b'b'], [b'C']])),
+      dict(
+          op=array_ops.where,
+          args=(ragged_factory_ops.constant_value([[True, False], [True]]),),
+          expected=[[0, 0], [1, 0]]),
       dict(
           op=math_ops.unsorted_segment_sum,
           kwargs={