Nothing can't be filtered

[BlueBaron]

TL,DR: Add the option to filter on splitters to "nothing", essentially making a 2->1 joiner.

What

1. If you want use a splitter to join two belts:
   - just use it as is.
2. If you want use a splitter to join two belts, and prevent items from exiting onto the left:
   - set the splitter to filter some nonsense item to the left lane. (Fish for instance)
   --or--
   - set the splitter to filter "nothing" to the left lane

Why
When making compact, tilable builds you some times need to block one output of a splitter. To prevent unwanted items contaminating the underground belt you managed to squeeze in in-front of the splitter. But you can't really block it formally apart from setting the filter to a nonsense item that "could" show up.

I want to be explicit, at work, at home and in Factorio

[Oktokolo]

An "anything" filter would be nice for inserters (and therefore miniloaders) too.
I would totally mod away the normal inserters and make "anything" the default filter on newly placed inserters.