BettiCurve: `not X` is not a safe way to test for emptiness

[mglisse]

gudhi-devel/src/python/gudhi/representations/vector_methods.py

Line 382 in e7b6d08

if not X:

(there is another one a few lines below)
I think this was meant as a short version of len(X)==0, but if X is a numpy array (which is perfectly valid), not does not have the intended behavior.

(replacing an empty list with a list that contains a single empty diagram also looks suspicious, but I did not look at the details)