Symfony version(s) affected: 4.4

Description
when importing routes from a resource file with a prefix, symfony will not match a valid route
with an allowable empty prefix unless the URL has a double / in it (//).

How to reproduce
Given this setting:

web:
  resource: '_routes.yaml'
  prefix: /{route_prefix?}
  requirements:
    route_prefix: valueA|valueB

and _routes.yaml:

product_list:
  path: '/brands/{brand_name}/{id}.html'
  controller: App\Controller\Brand\ProductController::index

the router will match:
/valueA/brands/{brand_name}/{id}.html and
/valueB/brands/{brand_name}/{id}.html but will not match
/brands/{brand_name}/{id}.html unless you provide the URL like this:
//brands/{brand_name}/{id}.html

Possible Solution
Allow for optional prefix values when importing routes.