Invalid consumer ID leads to exceptions

Problem/Motivation

If a request contains an invalid consumer ID (in the header or query parameter), a PHP fatal error is thrown:

Call to a member function uuid() on null in Drupal\consumers\Negotiator->negotiateFromRequest()

Steps to reproduce

Make a request without a consumer in the request and verify that everything is working as expected. Add ?consumerId=foo to the request URL and see that there is a fatal error. Now replace "foo" with the actual UUID of a consumer and verify that everything is fine again.

Proposed resolution

Fall back to the default consumer if an invalid one is passed. Since there is the concept of falling back to the default anyway (if none is passed explicitly) this seems to be in-line with the current semantics. Alternatively, a proper exception could be thrown, but that would be a change in documented behavior (not in actual behavior, because right now there is a fatal already).

Remaining tasks

User interface changes

API changes

Data model changes