Support creation of empty arrays

[martaiborra]

Right now blosc2.NDArrays cannot have a (0, ) shape because it eventually occurs a division by zero. This should be fixed.
The following example show this case works for dumpy but not for blosc2.

import numpy as np
import blosc2

a = np.array([])
print(a.shape)
print(a)

b = blosc2.asarray(a)
print(b.shape)
print(b)