Symfony 3.2 is able to parse yaml that looks like this:

factory.keyvalue.expirable: 
  {}
  # Default key/value expirable storage service to use.
  # @default keyvalue.database.expirable
  # default: keyvalue.database.expirable

Symfony 3.3+ it results in Uncaught Symfony\Component\Yaml\Exception\ParseException: Unable to parse at line 2 (near "{}")

Using git bisect shows that this break was introduced by #25241