* This Logger can be used to avoid conditional log calls.
* Logging should always be optional, and if no logger is provided to your
* library creating a NullLogger instance to have something to throw logs at
* is a good way to avoid littering your code with `if ($this->logger) { }`
class NullLogger extends AbstractLogger
* Logs with an arbitrary level.
* @throws \Psr\Log\InvalidArgumentException
public function log($level, $message, array $context = array())