Add return-typehint for generated buildRelations, to bring it in line with the base class typehint

[nederdirk]

Our 'symfony debug autoloader' was spamming messages for every generated TableMap-class on every request. This patch should bring the typehint of the generated TableMap implementations in line with the typehint of the base class

[nederdirk]

I wonder how this got missed

Do we have any other methods with missing return annotations etc?

None that I have noticed, but it's hard to tell with certainty. To be honest, I think the symfony debug autoloader is overly strict in this case, as it gives the reason 'the library you're using might in the future declare the return type as : void instead of @return void'

Also: since we're targeting php7.3+, we might as well specify : void instead of @return void here, right? that's compatible, see here:

>>> class A { public function a() {} }
>>> class B extends A { public function a(): void {} }
>>> $b = new B;
=> B {#2672}
>>> $b->a()
=> null

[dereuromark]

Yes, we should actually add consistent param/return types now everywhere possible for the next beta release. So we can actually have a more safe upgrade process from there on in the future - once all types are in place, no more fatal errors because suddenly the lib added types where there were none in the future.

We can make this a follow up PR.