Add support for null comparisons to conformance

matter/conformance/grammar/math.peg

@@ -1,6 +1,6 @@
 
 
-EquationTerm = (FeatureValue / ReferenceValue / IdentifierValue / NumberValue / '(' _ eq:EquationValue _ ')' { return eq, nil})
+EquationTerm = (FeatureValue / ReferenceValue / IdentifierValue / NumberValue / NullValue / '(' _ eq:EquationValue _ ')' { return eq, nil})
 
 EquationValue <- _ left:EquationMultiply  rest:( AddOp )* _ {
     if rest == nil {

matter/conformance/grammar/number.peg

@@ -31,3 +31,6 @@ Digits <- [0-9]+ {
 	return strconv.ParseInt(string(c.text), 10, 64)
 }
 
+NullValue <- "null" {
+	return NewNullValue(string(c.text)), nil
+}

matter/conformance/null.go

@@ -0,0 +1,58 @@
+package conformance
+
+import "fmt"
+
+type NullValue struct {
+	raw string
+}
+
+func NewNullValue(raw string) *NullValue {
+	return &NullValue{raw: raw}
+}
+
+func (fv NullValue) ASCIIDocString() string {
+	return fv.raw
+}
+
+func (fv NullValue) Description() string {
+	return fv.raw
+}
+
+func (fv *NullValue) Compare(context Context, other ComparisonValue, op ComparisonOperator) (bool, error) {
+	switch op {
+	case ComparisonOperatorEqual:
+		switch other.(type) {
+		case nil, *NullValue:
+			return true, nil
+		default:
+			return false, nil
+		}
+	case ComparisonOperatorNotEqual:
+		switch other.(type) {
+		case nil, *NullValue:
+			return false, nil
+		default:
+			return true, nil
+		}
+	default:
+		return false, fmt.Errorf("invalid operator with null value: %s", op.String())
+	}
+}
+
+func (fv NullValue) Value(context Context) (any, error) {
+	return nil, nil
+}
+
+func (fv *NullValue) Equal(ofv ComparisonValue) bool {
+	if ofv == nil {
+		return fv == nil
+	} else if fv == nil {
+		return false
+	}
+	_, ok := ofv.(*NullValue)
+	return !ok
+}
+
+func (fv NullValue) Clone() ComparisonValue {
+	return &NullValue{raw: fv.raw}
+}