Fix test that depended on snapshot version

A test case would fail to accurately capture the contents
of the layers.idx file unless the test project contained
at least one non-project snapshot dependency, which was only
true when the Spring Boot version was a snapshot.

See gh-23463

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/java/org/springframework/boot/maven/JarIntegrationTests.java

@@ -402,6 +402,7 @@ class JarIntegrationTests extends AbstractArchiveIntegrationTests {
 			while (line != null) {
 				if (line.startsWith("- ")) {
 					layer = line.substring(3, line.length() - 2);
+					index.put(layer, new ArrayList<>());
 				}
 				else if (line.startsWith("  - ")) {
 					index.computeIfAbsent(layer, (key) -> new ArrayList<>()).add(line.substring(5, line.length() - 1));