It seems that the error is here. Like if it does not accept `null` as third parameter.
That's weird... have a look at the full method signature:

```php
/**
     * Sends an authenticated API request to the path provided.
     * If the path provided is not an absolute URI, the base API Uri (service-specific) will be used.
     *
     * @param string|UriInterface $path
     * @param string              $method       HTTP method
     * @param array               $body         Request body if applicable (an associative array will
     *                                          automatically be converted into a urlencoded body)
     * @param array               $extraHeaders Extra headers if applicable. These will override service-specific
     *                                          any defaults.
     *
     * @return string
     */
public function request($path, $method = 'GET', $body = null, array $extraHeaders = array());
```

Does phpUnit create expectations based on docblocks?
Can you try by passing an empty array instead of that `null`?