Fix DCEGM/calcCrossPoints' return fields

[Mv77]

DCEGM's calcCrossPoints, which I implemented a while ago, has an annoying behavior.

The number of things it returns depends on whether or not it finds crossing points in the value function. This produces bugs when one calls the function, as one can not know how many things will be returned. This tiny PR fixes that issue.

[sbenthall]

Makes sense to make this change.
A nitpick: to keep the type signature consistent, the second returned element should be an ndarray not a list.

This can be done with np.array([]) or np.empty(0)

[Mv77]

Great point. Fixed in the previous commit.