Fix v4 issues

[nick-potts]

Currently, returning '' as the state fails, as it tries to render that as a column.

as per the getIcon method,

   public function getIcon(mixed $state): string | BackedEnum | null
    {
        if ($state === null) {
            return null;
        }

This is a more reliable way.

[Baspa]

Fixed in v3.0.2 in #53