ContainerBlock - add hasChildren method

Added ``ContainerBlock::hasChildren``, before it would return false. Now it can be used for the TTL cache cascading to the parents fixed in SonataBlockBundle PR: https://github.com/sonata-project/SonataBlockBundle/pull/96

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A