This pull request updates the type annotations for the grouped_by field in two usage-related model classes to allow for None values in the dictionary values. This change ensures that the models can represent cases where some grouping keys may not have a value.

Model updates for grouping support:

• Updated the grouped_by field in the GroupedUsage class in customer_usage.py to use Dict[str, str | None], allowing grouping values to be None.
• Updated the grouped_by field in the ProjectedGroupedUsage class in customer_projected_usage.py to use Dict[str, str | None], allowing grouping values to be None.