[PHP] Fix checking value of `Configuration::getAccessToken()`

Created by: jtreminio

Configuration::getAccessToken() returns an empty string, but API classes currently do a null check:

if ($this->config->getAccessToken() !== null)

This will always be true whenever an endpoint allows access token auth.

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changed title from Basicauth oauth to {+[PHP] Fix checking value of Configuration::getAccessToken()+}

added Client: PHP Feature: Authentication Issue: Bug labels

requested review from @root

closed

changed milestone to %6.0.0