bug#35222: 26.1; `read-command' documentation

[Stefan Monnier]

>> 1. The doc string does not say what happens if DEFAULT is not provided
>>    (so `nil') and the user enters empty input.  And what happens is not
>>    obvious.  Please add that information to the doc string.
> Um, can we declare the current behaviour a bug, and instead return nil
> in this case? That matches the current doc string (i.e., return
> DEFAULT-VALUE), and avoids the whole mess with the empty string symbol
> which is weird and pretty useless as far as I can tell.

Yes, it would seem to be more in line with the usual expected behavior.
The caller can get the current behavior by passing ## as the DEFAULT
argument, so there doesn't seem to be any good reason to return ## when
DEFAULT was nil.


        Stefan