ViewPageController::handle() should not ask for upcast route parameters

Problem/Motivation

Current code:

    $map = $route->hasOption('_view_argument_map') ? $route->getOption('_view_argument_map') : [];

    foreach ($map as $attribute => $parameter_name) {
      // Allow parameters be pulled from the request.
      // The map stores the actual name of the parameter in the request. Views
      // which override existing controller, use for example 'node' instead of
      // arg_nid as name.
      if (isset($map[$attribute])) {
        $attribute = $map[$attribute];
      }
      if ($arg = $route_match->getRawParameter($attribute)) {
      }
      else {
        $arg = $route_match->getParameter($attribute);
      }

      if (isset($arg)) {
        $args[] = $arg;
      }
    }

The issue with the above is that anything in '_view_argument_map' is controlled by us (via PathPluginBase) and will always have its keys on both the raw and the upcast parameters. Furthermore, argument and argument_validator plugins expect the raw parameters anyway.

The bug in the above code can manifest itself because of the falsy check on the raw parameter. If we have one with a value of '0', it will try to get the upcast parameter, which may break a validator that expects a raw value.

Steps to reproduce

Visit a view at foo/0 and see that argument validators receive an upcast parameter for 0, provided you have an upcast configured to do just that.

Proposed resolution

Either way, the fix is simple.

    foreach ($map as $attribute => $parameter_name) {
      // ...
      if (isset($map[$attribute])) {
        $attribute = $map[$attribute];
      }
      $arg = $route_match->getRawParameter($attribute);
      if (isset($arg)) {
        $args[] = $arg;
      } 
    }

This will only use the raw parameters and also strictly checks for NULL, rather than falsy. We'll need to update ViewPageControllerTest because it builds RouteMatch from a Request with only attributes set and no _raw_variables. If we set _raw_variables there, the test is fixed.

Remaining tasks

Create an MR with the above changes.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet