JPM/TJ: NumericDocValues.EMPTY will return 0 for docs, do not use.
TJ/JPM: as we allow keys to be optional a document's key value can be 0 in which case it must be ignored (instead of matching after all).
JPM: added NOT_YET_SUPPORTED_testJoinQueryOnOptionalKeyUniteResultsWithoutKey to make clear that joining on optional keys doesn't work in combination with unite.