GROOVY-8088: For @MapConstructor and @TupleConstructor we should prov…

…ide a default undefined value for better Java compatibility (closes groovy#498)

src/main/groovy/transform/MapConstructor.java

@@ -114,12 +114,12 @@
     /**
      * A Closure containing statements which will be prepended to the generated constructor. The first statement within the Closure may be "super(someArgs)" in which case the no-arg super constructor won't be called.
      */
-    Class pre();
+    Class pre() default Undefined.CLASS.class;
 
     /**
      * A Closure containing statements which will be appended to the end of the generated constructor. Useful for validation steps or tweaking the populated fields/properties.
      */
-    Class post();
+    Class post() default Undefined.CLASS.class;
 
     /**
      * Whether to include all fields and/or properties within the constructor, including those with names that are considered internal.

src/main/groovy/transform/TupleConstructor.java

@@ -267,12 +267,12 @@
      *
      * @since 2.5.0
      */
-    Class pre();
+    Class pre() default Undefined.CLASS.class;
 
     /**
      * A Closure containing statements which will be appended to the end of the generated constructor. Useful for validation steps or tweaking the populated fields/properties.
      *
      * @since 2.5.0
      */
-    Class post();
+    Class post() default Undefined.CLASS.class;
 }