don't null out array in HashMap when size drops to zero

As in ArrayList, we want to avoid thrashing in cases where the map is frequently emptied and refilled.

classpath/java/util/HashMap.java

@@ -66,8 +66,6 @@ public class HashMap<K, V> implements Map<K, V> {
   private void shrink() {
     if (array.length / 2 >= MinimumCapacity && size <= array.length / 3) {
       resize(array.length / 2);
-    } else if (size == 0) {
-      resize(0);
     }
   }