[NO ISSUE][COMP] Align ConstantExpression.toString() with IAObject

- user model changes: no
- storage format changes: no
- interface changes: no

Details:
- Align ConstantExpression.toString() with IAObject.toString()
  for NULL and MISSING values

Change-Id: Ia9a55a36f3d529836e1c24b7c22d3040b542e658
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/16643
Contrib: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Murtadha Hubail <mhubail@apache.org>
diff --git a/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java b/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java
index e9a2f2a..6f0cbba 100644
--- a/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java
+++ b/hyracks-fullstack/algebricks/algebricks-core/src/main/java/org/apache/hyracks/algebricks/core/algebra/expressions/ConstantExpression.java
@@ -129,7 +129,7 @@
 
         @Override
         public String toString() {
-            return "NULL";
+            return "null";
         }
 
         @Override
@@ -166,7 +166,7 @@
 
         @Override
         public String toString() {
-            return "MISSING";
+            return "missing";
         }
 
         @Override