[low priority] lazy `Phase` definitions can lead to unexpected errors with `Miller`

[argerlt]

I noticed while working on #645 that the following snippet of code gives no error:

import orix.crystal_map as ocm
import orix.vector as ove
ph = ocm.Phase()
m = ove.Miller.random(10, phase=ph)

but then printing m raises the following error:

AttributeError: 'NoneType' object has no attribute 'name'

Which, tracing back, occurs because giving no point group or space group to Phase means there is no Phase.point_group.name attribute to use for Phase.name

One solution is just to add the following logic to Phase.__init__ so that a phase always has a symmetry:

        if space_group is None and point_group is None:
            warnings.warn("No symmety given, using point group C1")
            point_group = C1

However, maybe we want to be able to have empty phases?

Either way though, something should be done to ensure phases passed into a crystal vector have a defined symmetry, either by setting a default phase symmetry or a if/then/else check in the Miller class.