Generic binding with command is an infinite loop

As mentioned in #5528, if I do

```fish
bind "" true
```

and press any key, it'll send fish spinning.

This is because, as the code says:

```c++
input_common_next_ch(c);
input_mapping_execute_matching_or_generic(allow_commands);
// Regarding allow_commands, we're in a loop, but if a fish command
// is executed, R_NULL is unread, so the next pass through the loop
// we'll break out and return it.
```

We've read something, but (for reasons I don't understand) we don't want to execute commands yet, so we put an R_NULL at the front of the input queue.

But we stay in the loop, so the next time we read that R_NULL, which isn't bound to anything so we try the generic binding, which has a command, so instead of executing it we put an R_NULL, and then we loop again.

I'm not sure if this was supposed to _return_ R_NULL, as a value, or if R_NULL wasn't supposed to trigger _any_ binding otherwise, or what.

Just changing it to return R_NULL here instead of putting it back doesn't work.

My main problem with fixing this is that I have a very fuzzy idea of what all of this is supposed to _mean_. What's an "R_NULL"? Is that a separator, so that we can't confuse "abc" with "abcd", because it's really "abcR_NULLd"?

Is it a "yield marker", so that we then break out of the loop to go do other things?

Why aren't commands allowed here? When will they be allowed again?