private ResultList evaluateWithAtLeastOneNullSide(Object lhs, Object rhs) {
        if (operator.equals("==") || operator.equals("=")) { // Glob (=) operator falls back to equality for non-strings
            return ResultList.fromBoolean(lhs == rhs);
        } else if (operator.equals("!=")) {
            return ResultList.fromBoolean(lhs != rhs);
        } else {
            return new ResultList(Result.INVALID);
        }
    }
Evaluates a binary comparison where one or both operands are null.
Boolean outcomes are only defined for (in)equality relations, all others
return Result.INVALID.

Precondition: lhs AND/OR rhs is null.
document/src/main/java/com/yahoo/document/select/rule/ComparisonNode.java