title: How to return the equivalent of "None" in Tensorflow
body: <p>If I have some branching operation in TensorFlow, how can I return a None tensor in one branch, and a filled tensor in the other?</p>
  
  <p>For example:</p>
  
  <pre><code>tensor_result = tf.cond(
      pred=tf.less(0, 1),
      fn1=...,  # here I would like to return None
      fn2=tf.constant([1, 2, 3]))
  </code></pre>
  
  <p>And then <code>tensor_result</code> can be tested for being None later in the graph.</p>
  
  <p>Is there currently any way of doing this? Currently I am filling a tensor with NaNs, but I imagine this isn't very efficient.</p>