DescriptorArray.variance = None crashes instead of clearing variances

[rozyczko]

Finding V-H1 in DEEP_ANALYSIS.md (parent #261).

src/easyscience/variable/descriptor_array.py:320-335 — the final assignment sits outside the if variance is not None: guard:

        # Values must be floats for optimization
        self._array.variances = variance.astype('float')   # None.astype -> AttributeError

Passing None (explicitly allowed by the guard and the docstring) raises AttributeError. There is currently no way to clear variances on an array via this property. The sibling error setter (lines 373-388) handles None correctly in an else branch — mirror that.