Reference binding to nullptr in `RaggedTensorToVariant`
### Impact
An attacker can cause undefined behavior via binding a reference to null pointer in `tf.raw_ops.RaggedTensorToVariant`:
```python
import tensorflow as tf
tf.raw_ops.RaggedTensorToVariant(
rt_nested_splits=[],
rt_dense_values=[1,2,3],
batched_input=True)
```
The implementation
https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/core/kernels/ragged_tensor_to_variant_op.cc#L129 has an incomplete validation of the splits values, missing the case when the argument would be empty.
### Patches
We have patched the issue in GitHub commit be7a4de6adfbd303ce08be4332554dff70362612
https://github.com/tensorflow/tensorflow/commit/be7a4de6adfbd303ce08be4332554dff70362612.
The fix will be included in TensorFlow 2.6.0. We will also cherrypick this commit on TensorFlow 2.5.1, TensorFlow 2.4.3, and TensorFlow 2.3.4, as these are also affected and still in supported range.
### For more information
Please consult our security guide
https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md for more information regarding the security model and how to contact us with issues and questions.
### Attribution
This vulnerability has been reported by members of the Aivul Team from Qihoo 360.