Fix "using a tf.Tensor as a Python bool is not allowed"

[willduan]

Since this issue is still not fixed in the past nine months, so I refix it

TypeError: Using a tf.Tensor as a Python bool is not allowed. Use if t is not None: instead of if t: to test if a tensor is defined, and use TensorFlow ops such as tf.cond to execute subgraphs conditioned on the value of a tensor.

So I fix it to make it more robust.

[aymericdamien]

LGTM! Thanks for updating that old issue. And sorry that I missed your PR..