Deprecated function: parse_str(): Passing null to parameter #1 ($string) of type string is deprecated in Drupal\redirect\EventSubscriber\RedirectRequestSubscriber->onKernelRequestCheckRedirect

This is one of the simplest fixes so probably okay. I wonder why we are not doing $request_query = $request->query->all(); - however both Symfony's query parameter bag and parse_str() can result in changing the initial query string.

Hmmm - actually given that \Symfony\Component\HttpFoundation\Request::getQueryString() calls \Symfony\Component\HttpFoundation\Request::normalizeQueryString() I think we should take the opportunity to change this to $request_query = $request->query->all(); as that is guaranteed to return an array and then we don't have to parse the query params multiple times.

There used to be some really nasty bugs around caching and redirect loops where a normalized query string was different but caching uses the other one. But, I think that was actually all in \Drupal\redirect\EventSubscriber\RouteNormalizerRequestSubscriber::onKernelRequestRedirect(), this here is just old code, hasn't changed since 2014. So yes, sounds fine to do that.

Simple change. Looks good.

Thanks, merged.

There's one more 8.1 related test problem that people following this might be interested in fixing (see what I did there?): #3253718: Fix PHP 8.1 test fails