Ticket: #2517 (review)

We currently return false in the object factories when no object could be created. Two examples for posts:

• See Timber\Factory\PostFactory::from()
• See Timber\Timber::get_post()

We should change this to return null, because

• Being able to use the null coalescing operator on Timber\Timber::get_post() is a big plus.
• WordPress’s get_post() function also returns null if no post can be found.

This does not only apply to posts, but also other factories.