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

[SnowWalkerJ]

At the current version of tflearn, using some modules could end up with:

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 replaced some judgements like "if b:" to "if b is not None" to avoid this.

[aymericdamien]

Thanks for fixing this issue! Unfortunately the PR breaks due to the latest tflearn update. If you got time, would you mind rebase your code? Thanks!

[zxydi1992]

Can someone fix that?

[aldanor]

This should be closed ^