Avoid input None for `a` and `b` in `tf.test.TestCase.assertAllClose`

[DarrenZhang01]

As titled, TF assertAllClose API does not support None inputs. But some attributes of x_dict and y_dict are None sometimes in test_utils.py. Need a way to treat this case separately.

[DarrenZhang01]

Solved in a2575d8