take account 'none' for box-shadow

[wojtask9]

Fixes #26478

[MartijnCuppens]

I've just tested this and it LGTM. I was a bit worried the mixin couldn't handle multiple values because sass lists can be separated by as well spaces as commas (https://hugogiraudel.com/2013/07/15/understanding-sass-lists/#sass-list-fun-facts), but that doesn't seemed to be a problem

[danijelGombac]

@MartijnCuppens in scss/_buttons.scss

&.disabled,
  &:disabled {
    opacity: $btn-disabled-opacity;
    @include box-shadow(none); // this is not take into account
}

[wojtask9]

indeed. I missed that in my PR.

there is another place where it breaks
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_button-group.scss#L102

Possible solutnions:

1. change this to box-shadow(unset)
2. change mixin box-shadow in that way if there is only one none shadow then apply to box-shadow property