fix filtering when groupFilter exists

this fixes an issue where, when a filter text is entered and a group becomes empty, it crashes  with a `TypeError: groups[groupValue] is not iterable (cannot read property undefined)`

src/Select.svelte

@@ -281,7 +281,7 @@
         const sortedGroupedItems = [];
 
         groupFilter(groupValues).forEach((groupValue) => {
-            sortedGroupedItems.push(...groups[groupValue]);
+            if (groups[groupValue]) sortedGroupedItems.push(...groups[groupValue]);
         });
 
         return sortedGroupedItems;