check for server.clients undefined

[tinchoz49]

Hi,

I found that if I have the websocket option clientTracking in false it throws an error during the server.close() because server.clients is undefined and the plugin expect always an iterable.

Checking for the server.clients undefined fix the issue.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm