Filter by `None`/`Any` for weight in issues API

See &454 for filtering on different fields in different scenarios.

• ?weight=None: Return results where there is no weight entered.
• ?weight=Any: Return results where that is a weight entered.

Applies to these API endpoints:

• List project issues
• List group issues
• List dashboard issues (i.e. https://docs.gitlab.com/ee/api/issues.html#list-issues)