[GH-564] Fix a wrong return type

This issue is created automatically through a Github pull request on behalf of BenMorel:

Url: https://github.com/doctrine/doctrine2/pull/564

Message:

`BasicEntityPersister::executeInserts()` is documented as:

@return array An array of any generated post-insert IDs. This will be an empty array
if the entity class does not use the IDENTITY generation strategy.

This PR fixes an empty `return;` that contradicted the documented return type.