cluster: default value of exitedAfterDisconnect changed to false

The defaul value of worker.exitedAfterDisconnect is undefined. If the
worker dies on its own the value is changed to false and doesn't
remains undefined. While the documentation just mentions 'Set by
calling .kill() or .disconnect(). Until then, it is undefined.'.
Making the default value as false in worker.js

Fixes: nodejs#28837
Refs: nodejs#3743

lib/internal/cluster/worker.js

@@ -16,7 +16,7 @@ function Worker(options) {
   if (options === null || typeof options !== 'object')
     options = {};
 
-  this.exitedAfterDisconnect = undefined;
+  this.exitedAfterDisconnect = false;
 
   this.state = options.state || 'none';
   this.id = options.id | 0;