`fftn` of zero size arrays

[vtavana]

Following example returns an empty array

import mkl_fft.interfaces as mfi, numpy
a=numpy.ones((2,0,4))
mfi.numpy_fft.fftn(a)
# array([], shape=(2, 0, 4), dtype=complex128)

mfi.numpy_fft.fftn(a, axes=(1,))
# array([], shape=(2, 0, 4), dtype=complex128)

while stock NumPy raises a ValueError

import numpy
a=numpy.ones((2,0,4))
numpy.fft.fftn(a)
# ValueError: Invalid number of FFT data points (0) specified.

numpy.fft.fftn(a, axes=(1,))
# ValueError: Invalid number of FFT data points (0) specified.

[vtavana]

was resolved as part of #139