Allow null values as tag attributes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

see also https://github.com/symfony/DependencyInjection/commit/3a194b718d6c948fcfa346798510184841e07335#commitcomment-4096394

fixes a regression see https://travis-ci.org/symfony-cmf/RoutingBundle/jobs/11362114#L165