Adding not-nullable search methods to ObjectRepository

I'm currently in the process of improving my codebase with static analysis (PHPStan in this case), and I've found an interesting use case to improve the `ObjectRepository` interface: I find it pretty annoying have to specify/check each time that I don't expect null values from my `find()`/`findOneBy()` methods etc, so it would be nice if there was some `get*()` twin methods for which a nullable return would not be allowed.

This is obviously a BC, so it should target 3.0, hence leveraging nullable types from PHP 7.1 it would make really sense.... What do you think?

PS: since we already have doctrine/common#826 doctrine/common#738 etc, maybe we can create a 3.0 milestone?