## Bug description

When calculating Trends results with an aggregated (e.g. single output, like big number) display type, WAU/MAU don't work. This is because we have an `ACTIVE_USERS_SQL` query for time series display types, but we don't have a version of this for aggregated display types. (See ZEN-1043.)

## How to reproduce

1. Create an insight with a single WAU or MAU series.
2. Switch display type to "Number".
3. Open the "Debug ClickHouse queries" modal from the command palette.
4. See `COUNT(*)` instead of `COUNT(DISTINCT person_id)`.

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posthog/queries/person_query.py
"Returns person table fields this query exposes"
        return [alias for column_name, alias in self._get_fields()]
    @property
    def is_used(self):
        "Returns whether properties or any other columns are actually being queried"
        if any(self._uses_person_id(prop) for prop in self._filter.property_groups.flat):
            return True
        for entity in self._filter.entities:
            if entity.math in COUNT_PER_ACTOR_MATH_FUNCTIONS or any(
                self._uses_person_id(prop) for prop in entity.property_groups.flat
            ):
                return True
        return len(self._column_optimizer.person_columns_to_query) > 0
    def _uses_person_id(self, prop: Property) -> bool:
        return prop.type in ("person", "static-cohort", "precalculated-cohort")
    def _get_fields(self) -> List[Tuple[str, str]]: