added null support to parsing

src/main/java/jef/expressions/NullExpression.java

@@ -0,0 +1,19 @@
+package jef.expressions;
+
+public class NullExpression extends ConstantExpression {
+    public static final NullExpression INSTANCE = new NullExpression();
+
+    private NullExpression() {
+        super(null);
+    }
+
+    @Override
+    public Type getType() {
+        return Type.NULL;
+    }
+
+    @Override
+    public String toString() {
+        return "NULL";
+    }
+}