Fixed hdf5plugin.Bitshuffle to avoid raising ValueError when lz4=False

[psteinb]

Just ran into this problem and would like to avoid other users to see a ValueError if they do:

f = h5py.File('test.h5', 'w')
f.create_dataset(
    'bitshuffle_with_lz4',
    data=numpy.arange(100),
    **hdf5plugin.Bitshuffle(nelems=0, lz4=False, cname='zstd'))
f.close()

[t20100]

Thanks for the fix!