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

[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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[jtreminio]

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko @renepardon

[wing328]

LGTM too. @ybelenko any concern merging this PR?

[ybelenko]

I would move logic from API controllers to Configuration class, to make possible cover it with tests. But we're far away from that point. This changes looks OK as temp fix. It wasn't broken, empty access token gives no access anyway.

We can merge this PR.

[jtreminio]

@ybelenko Thank you for merging. Just a quick update - this actually fixes a bug, where if an endpoint allows both Basic Auth and Bearer Token, but the user is only using Basic Auth, the previous code would override to use an empty Bearer Token.