RankedReducer returns null in certain cases instead of throwing an UnsatisfiedReductionException

[ljnelson]

microbean-bean/src/main/java/org/microbean/bean/RankedReducer.java

Lines 66 to 68 in a9cf9e8

	if (elements == null || elements.isEmpty()) {
	return null;
	} else if (elements.size() == 1) {

I think this return null should actually invoke the failure handler with an empty list.