Add empty collection handling to RecursiveCollectionLineAggregator

Cherry-picked from dbf489e

Author: florian-serraille

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/file/transform/RecursiveCollectionLineAggregator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2024 the original author or authors.
+ * Copyright 2006-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -56,6 +56,9 @@ public void setLineSeparator(String lineSeparator) {
 
 	@Override
 	public String aggregate(Collection<T> items) {
+		if (items.isEmpty()) {
+			return "";
+		}
 		StringBuilder builder = new StringBuilder();
 		for (T value : items) {
 			builder.append(delegate.aggregate(value)).append(lineSeparator);

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/file/transform/RecursiveCollectionLineAggregatorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2024 the original author or authors.
+ * Copyright 2006-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,6 +33,12 @@ class RecursiveCollectionLineAggregatorTests {
 
 	private final RecursiveCollectionLineAggregator<String> aggregator = new RecursiveCollectionLineAggregator<>();
 
+	@Test
+	void testSetDelegateAndPassEmptyCollection() {
+		aggregator.setDelegate(item -> "bar");
+		assertEquals("", aggregator.aggregate(Collections.emptyList()));
+	}
+
 	@Test
 	void testSetDelegateAndPassInString() {
 		aggregator.setDelegate(item -> "bar");