CVE-2021-37643: Null pointer dereference in `MatrixDiagPartOp`

If a user does not provide a valid padding value to tf.raw_ops.MatrixDiagPartOp, then the code triggers a null pointer dereference (if input is empty) or produces invalid behavior, ignoring all values after the first:

import tensorflow as tf

tf.raw_ops.MatrixDiagPartV2(
input=tf.ones(2,dtype=tf.int32),
k=tf.ones(2,dtype=tf.int32),
padding_value=[])

Although this example is given for MatrixDiagPartV2, all versions of the operation are affected.