This PR was merged into the 3.3-dev branch.

Discussion
----------

[DI] Allow ~ instead of {} for services in Yaml

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Now we can omit the class name, Yaml config can look like this:
```yaml
services:
    AppBundle\Something: {}
```

While this works, the consistent way in Symfony for such cases is to use `~`. This PR allows to use `~` as well, ending up with:
```yaml
services:
    AppBundle\Something: ~
```

Commits
-------

28a1b5a Allow setting services as null