[rrd4j] Fix unit retrieval for group items (openhab#17054)

Reported on the forum

Signed-off-by: Jan N. Klug <github@klug.nrw>

bundles/org.openhab.persistence.rrd4j/src/main/java/org/openhab/persistence/rrd4j/internal/RRD4jPersistenceService.java

@@ -443,6 +443,9 @@ public Iterable<HistoricItem> query(FilterCriteria filter) {
  Unit<?> unit = null;
  try {
  item = itemRegistry.getItem(itemName);
+ if (item instanceof GroupItem groupItem) {
+ item = groupItem.getBaseItem();
+ }
  if (item instanceof NumberItem numberItem) {
  // we already retrieve the unit here once as it is a very costly operation,
  // see https://github.com/openhab/openhab-addons/issues/8928
@@ -631,11 +634,14 @@ public ConsolFun getConsolidationFunction(RrdDb db) {
  }
  }
 
+ /**
+ * Get the state Mapper for a given item
+ *
+ * @param item the item (in case of a group item, the base item has to be supplied)
+ * @param unit the unit to use
+ * @return the state mapper
+ */
  private <Q extends Quantity<Q>> DoubleFunction<State> toStateMapper(@Nullable Item item, @Nullable Unit<Q> unit) {
- if (item instanceof GroupItem groupItem) {
- item = groupItem.getBaseItem();
- }
-
  if (item instanceof SwitchItem && !(item instanceof DimmerItem)) {
  return (value) -> OnOffType.from(value != 0.0d);
  } else if (item instanceof ContactItem) {