Don't hard-code allow_None to True in Array

[maximlt]

Fixes #583

This is effectively a breaking change. The code below used to work fine, while after this change p.a = None will raise an error.

import param, numpy as np

class P(param.Parameterized):
    a = param.Array(np.zeros((2, 2)))

p = P()

p.a = None

[jlstevens]

Simple change and looks correct to me. Based on the discussion in #583, this is a breaking change but one we think makes sense for 2.0.

[maximlt]

Thanks for the review! Merging.