events: use null assignment instead of deleting property

lib/events.js

@@ -226,7 +226,7 @@ EventEmitter.prototype.removeListener = function(type, listener) {
     if (position < 0) return this;
     list.splice(position, 1);
     if (list.length == 0)
-      delete this._events[type];
+      this._events[type] = null;
 
     if (this._events.removeListener) {
       this.emit('removeListener', type, listener);
@@ -234,7 +234,7 @@ EventEmitter.prototype.removeListener = function(type, listener) {
   } else if (list === listener ||
              (list.listener && list.listener === listener))
   {
-    delete this._events[type];
+    this._events[type] = null;
 
     if (this._events.removeListener) {
       this.emit('removeListener', type, listener);