None getting wrapped in list

[kjaym]

@maxhutch https://github.com/CitrineInformatics/pypif/blob/develop/pypif/obj/common/value.py#L72-L73 This is causing errors when it wraps a None in a list. That is not allowed. I think that checking for None is the setter removes the need for this https://github.com/CitrineInformatics/pypif/blob/develop/pypif/obj/common/value.py#L37-L38.