Get the user dict from cache or query it from the platform if missing.

async def _get_user(self):
        """
        Get the user dict from cache or query it from the platform if missing.
        """

        if self._cache is None:
            try:
                self._cache = \
                    await self.facebook.get_user(self.fbid, self.page_id)
            except PlatformOperationError:
                self._cache = {}
        return self._cache