Do not allocate a new ValueString in Parameter.optimize()

h2/src/main/org/h2/expression/Parameter.java

@@ -85,8 +85,8 @@ public class Parameter extends Expression implements ParameterInterface {
     @Override
     public Expression optimize(Session session) {
         if (session.getDatabase().getMode().treatEmptyStringsAsNull) {
-            if (value instanceof ValueString) {
-                value = ValueString.get(value.getString(), true);
+            if (value instanceof ValueString && value.getString().isEmpty()) {
+                value = ValueNull.INSTANCE;
             }
         }
         return this;