Missing request info throws additional error on getRealMethod

Following the issue on https://github.com/drush-ops/drush/issues/3380 which states a Kernel error 'Container cannot be saved to cache.' might occur when invoking drush updb we throw an additional error when asking for getRealMethod on 'null' (empty request stack).

Error: Call to a member function getRealMethod() on null in Drupal\jsonlog\Logger\JsonLog->prepareLog()

Although being an edge case we should check if we're dealing with a http-request before asking getRealMethod() on the RequestStack. Maybe we should also inject this RequestStack via DI instead of making the static call to Drupal::request().

Proposed solution is not trying to log the real method (GET,POST,..) if the actual request from context is unknown meaning an empty $context['request_uri'] or Drupal::request().