Bugfix: return undefined, not null, to indicate absence in the map.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@4386 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/util/Hashtable.as

@@ -78,7 +78,7 @@ public class Hashtable
         var e :Entry = (_entries[index] as Entry);
         while (true) {
             if (e == null) {
-                return null;
+                return undefined;
             }
             if (e.hash == hash && e.key.equals(hkey)) {
                 return e.value;