Don't recommend setting user-agent to undefined

Not to confuse with `got.mergeOptions()`, where it keeps the default value.
Use `null` instead.

readme.md

@@ -749,7 +749,7 @@ const createTestServer = require('create-test-server');
 
 ### User Agent
 
-It's a good idea to set the `'user-agent'` header so the provider can more easily see how their resource is used. By default, it's the URL to this repo. You can omit this header by setting it to `null` or `undefined`.
+It's a good idea to set the `'user-agent'` header so the provider can more easily see how their resource is used. By default, it's the URL to this repo. You can omit this header by setting it to `null`.
 
 ```js
 const got = require('got');