public function whereNull($field, string $op = ''): self
    {
        return $this->where($this->prepareField($field) .' IS NULL', null, $op);
    }
public function whereNull($field, string $op = ''): self
    {
        return $this->where($this->prepareField($field) .' IS NULL', null, $op);
    }
public function whereNull ( $ field , string $ op = '' ) : self { return $ this -> where ( $ this -> prepareField ( $ field ) . ' IS NULL' , null , $ op ) ; }
Where null.
@param  string|array|Builder $field
@param  string               $op
@return self
Where null .