Null location Strings can cause problems with GroupedSplit serialization

If the location is a non-null array, and one of the elements is null, and the GroupedSplit is the same as the raw split - the groupedSplit throws an NPE during serialization.

This isn't necessarily a bug - since the original split location should not be null. However, there's parts of the code which ignore such null locations (and there's a test for this), while others don't. This should be done consistently.