[solved] list of list with one or more empty one

[Gribouillis]

Mprime = [[0] if n == [] else n for n in M]

Try this perhaps

Mprime = [n or [0] for n in M]

or if you dont mind having shared lists

x = [0]
Mprime = [n or x for n in M]

Or don't build a list, use a generator

from itertools import chain
x = [0]
Mprime =  Mprime = np.fromiter(chain.from_iterable(n or x for n in M), dtype=np.uint32)
Mprime = Mprime.reshape((Mprime.size, 1))