Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / vendor / symfony / yaml / Tests / Fixtures / legacyNullMappingKey.yml

--- %YAML:1.0
test: Miscellaneous
spec: 2.21
yaml: |
  null: ~
php: |
  array(
    '' => null,
  )