Fix bug in _battery_manager.py

A function's existence was being checked rather than its (boolean)
result.

This wasn't an issue in practice, because ever since we've had the
PowerManager, the PowerDistributor has not received requests when
there's no data.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/sdk/actor/power_distributing/_component_managers/_battery_manager.py

@@ -446,9 +446,9 @@ def _get_battery_inverter_data(
         # This should be handled by BatteryStatus. BatteryStatus should not return
         # this batteries as working.
         if not all(
-            self._battery_caches[bat_id].has_value for bat_id in battery_ids
+            self._battery_caches[bat_id].has_value() for bat_id in battery_ids
         ) or not all(
-            self._inverter_caches[inv_id].has_value for inv_id in inverter_ids
+            self._inverter_caches[inv_id].has_value() for inv_id in inverter_ids
         ):
             _logger.error(
                 "Battery %s or inverter %s send no data, yet. They should be not used.",