It looks like WordPress doesn’t ignore empty post__in
param. So you’ll have to do this yourself like this…
$ids="1,2,3";
if ( $ids ) {
$arr = explode("," ,$ids);
$args = array(
'posts_per_page' => $number,
'post__in' => $arr
);
} else {
$args = array(
'posts_per_page' => $number
);
}