def remove_empty_tags(self, tags=None):
        """
        Tags whose value is set to None shall be removed from tags.
        :param tags: Tags which are to be processed.
        If None, tags found in self._requirements are used.
        :return: If tags is not None, returns dict with processed tags. Else returns None.
        """
        new_tags = {}
        old_tags = tags if tags else self.get("tags")
        for tag in old_tags.keys():
            if old_tags[tag] is not None:
                new_tags[tag] = old_tags[tag]
        if not tags:
            self._requirements["tags"] = new_tags
            return None
        return new_tags