Use correct instance of redis to getLastError

Same as #38358 but handles also the dbindex error correctly.

The $redis is only set in the tests, the correct redis instance is available under $this->connection.

Reproduce: give a false AUTH to the redis:

Before:

[Error]
Call to a member function getLastError() on null

Now:

[Symfony\Component\Messenger\Exception\InvalidArgumentException]
Redis connection failed: ERR Client sent AUTH, but no password is set

In 5.x this class was moved hope git will handle it correctly.