fix(ui5-avatar-group): prevent console error when no items are present

Added a null-check in the `firstAvatarSize` getter to prevent a
TypeError when the component is initialized without items. This ensures
that the component behaves correctly and does not throw errors in the
console when it is first loaded or when no items are present.

Fixes #9581

packages/main/src/AvatarGroup.ts

@@ -369,7 +369,7 @@ class AvatarGroup extends UI5Element {
 	}
 
 	get firstAvatarSize() {
-		return this.items[0].size;
+		return this.items[0]?.size ?? AvatarSize.S;
 	}
 
 	get classes() {