bug#70795: 30.0.50; inconsistent warning-display-at-bottom display-buffer-alist behavior

[Juri Linkov]

> (add-to-list 'display-buffer-alist
>                `(,(rx (or "*Warnings*" "*compilation*"))
>                  display-buffer-no-window
>                  (allow-no-window . t)))
>
> then warnings are added to the hidden *Warnings* buffer.

This is because this case doesn't set 'warning-series',
so when window is nil, it doesn't fail at anoter place
while calling 'set-window-start':

  (when (and (markerp warning-series)
             (eq (marker-buffer warning-series) buffer))
    (set-window-start window warning-series))

Ok, I will add only checks for nil window.
But won't add (allow-no-window . t) to the caller of
'display-buffer' in 'display-warning' to not encourage
hiding the warning buffer.