Allow earlier optional parameters in routes to be empty

Problem/Motivation

I'm trying to add something like /ajax_get_jobs/en/{langCode}/{searchTerm}/{jobTypes}/{units}/{page} in a custom route. However, if I leave one of the parameters in the middle (searchTerm for example) empty (for example /ajax_get_jobs/en//Full-time/Central it no longer works even though there are defaults defined for all of the parameters.

Seems that using vanilla Symphony adding ! in front of each of the parameter definitions should work per Symphony documentation, but the exclamation mark doesn't seem to work in Drupal at all.

The Drupal routing documentation isn't too specific on this. It does specify that ”the first item of the path cannot be an argument, and must be a string. You can also define optional parameters at the end of your path (See 'Optional Parameters' on this page).”, which could mean that all empty optional parameters must be in the end of the path, but could also mean just that the optional parameters must be defined in the end of the route.

It also specificies that ”unlike generic Symfony routes, Drupal requires that a slug occupies a complete path part”, but that also doesn't mean that a path part can't be empty (ie. nothing between the slashes).

Steps to reproduce

Create a custom module with a routing definition like

example_module.job_data:
  path: '/ajax_get_jobs/{langCode}/{searchTerm}/{jobTypes}/{units}/{page}'
  defaults:
    _controller: '\Drupal\example_module\Controller\JobController::getJobs'
    searchTerm: NULL
    jobTypes: NULL
    units: NULL
    page: 0
  requirements:
    _permission: 'access content'

Try accessing /fi/ajax_get_jobs/fi//Full-time/Central. The request doesn't match the route, unlike if you have a searchTerm that's not empty.

Proposed resolution

Either allow an empty optional parameter in the middle of the route or clearly document that it isn't possible.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet