How to know if dynamic tensor returned by "tf.boolean_mask" is empty or not?
There are multiple ways to solve this, basically you are trying to identify a null tensor. 
Possible solutions can be:

is_empty = tf.equal(tf.size(boolean_tensor), 0). If not empty it will give false
Count non zeros number using tf.count_nonzero(boolean_tensor)
By simply printing the tensor and checking the vaules
tf.boolean_mask(tensor, mask) => returns (?, 4)
How do I check if the returned tensor by boolean_mask is empty or not?