Prevent deprecation notice when AdminInterface.isCurrentChild is implemented

[jorrit]

Subject

Even when a child admin instance implements the isCurrentChild() method, getCurrentChildAdmin() will try to call the deprecated method getCurrentChild() anyway, causing this deprecation notice:

User Deprecated: The Sonata\AdminBundle\Admin\AbstractAdmin::getCurrentChild() method is deprecated since version 3.65 and will be removed in 4.0. Use Sonata\AdminBundle\Admin\AbstractAdmin::isCurrentChild() instead.

I changed the code such that getCurrentChildAdmin() only calls getCurrentChild() when isCurrentChild() is not implemented.

I am targeting this branch, because it's a backwards compatible improvement.

Changelog

### Fixed
- Superfluous deprecation notice in `getCurrentChild()` when calling `getCurrentChildAdmin()`.

[jordisala1991]

Thank you @jorrit