GH-4769 Surfaced a bug in the comparison of null values (UNDEF)

Signed-off-by: Jerven Bolleman <jerven.bolleman@sib.swiss>

Author: JervenBolleman

core/queryalgebra/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/util/QueryEvaluationUtil.java

@@ -107,7 +107,7 @@ public static boolean compare(Value leftVal, Value rightVal, CompareOp operator)
 
 	public static boolean compare(Value leftVal, Value rightVal, CompareOp operator, boolean strict)
 			throws ValueExprEvaluationException {
-		if (leftVal.isLiteral() && rightVal.isLiteral()) {
+		if (leftVal != null && leftVal.isLiteral() && rightVal != null && rightVal.isLiteral()) {
 			// Both left and right argument is a Literal
 			return compareLiterals((Literal) leftVal, (Literal) rightVal, operator, strict);
 		} else {