Fix EntityOwnerInterface::getOwnerId() documentation

Hm, really?

The code for all the methods that override this interface method looks basically like this:

return $this->get('uid')->target_id;

Can this really be NULL?

Can this really be NULL?

yep, it can!

OK then.

If we're going to improve the documentation to say this returns a null sometimes shouldn't we take the time to document why.

<stefan_r> pfrenssen: do we know when the uid field on an entity might be NULL?
<stefan_r> I grepped for usages of this and it looks like it's when the field has not been set, or was explicitly set to NULL in case there was no author, or in case we remove it so non-admin users can send it over REST
<pfrenssen> stefan_r: yes when it has been created programmatically during a test for example
<pfrenssen> stefan_r: nothing stops you from calling Entity::create() and not setting the uid

Thanks!

Committed b7a8acf and pushed to 8.0.x. Thanks!