bug#10987: ibuffer: remove "No Buffers" error

[Chong Yidong]

Christopher Schmidt <christopher@ch.ristopher.com> writes:

> I use ibuffer-never-show-predicates to reduce the list of buffers in
> ibuffer.  This works fine whilst there are buffers for ibuffer to
> display.  If there are no buffers, ibuffer will error (and will leave an
> unusable ibuffer-buffer behind).  What the point behind this error?  I
> think that no buffers is a perfectly legitimate situation.

Thanks.  I committed your patch to trunk.