Include empty layers when listing and extracting

Fixes gh-21301

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/test/java/org/springframework/boot/jarmode/layertools/ExtractCommandTests.java

@@ -69,10 +69,11 @@ class ExtractCommandTests {
 	@Test
 	void runExtractsLayers() throws Exception {
 		this.command.run(Collections.emptyMap(), Collections.emptyList());
-		assertThat(this.extract.list()).containsOnly("a", "b", "c");
+		assertThat(this.extract.list()).containsOnly("a", "b", "c", "d");
 		assertThat(new File(this.extract, "a/a/a.jar")).exists();
 		assertThat(new File(this.extract, "b/b/b.jar")).exists();
 		assertThat(new File(this.extract, "c/c/c.jar")).exists();
+		assertThat(new File(this.extract, "d")).isDirectory();
 	}
 
 	@Test
@@ -119,7 +120,7 @@ class ExtractCommandTests {
 
 		@Override
 		public Iterator<String> iterator() {
-			return Arrays.asList("a", "b", "c").iterator();
+			return Arrays.asList("a", "b", "c", "d").iterator();
 		}
 
 		@Override

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/test/java/org/springframework/boot/jarmode/layertools/IndexedLayersTests.java

@@ -52,7 +52,7 @@ class IndexedLayersTests {
 	@Test
 	void iteratorReturnsLayers() throws Exception {
 		IndexedLayers layers = new IndexedLayers(getIndex());
-		assertThat(layers).containsExactly("test", "application");
+		assertThat(layers).containsExactly("test", "empty", "application");
 	}
 
 	@Test

spring-boot-project/spring-boot-tools/spring-boot-jarmode-layertools/src/test/resources/org/springframework/boot/jarmode/layertools/test-layers.idx

@@ -1,6 +1,7 @@
 - "test":
   - "BOOT-INF/lib/a.jar"
   - "BOOT-INF/lib/b.jar"
+- "empty":
 - "application":
   - "BOOT-INF/classes/Demo.class"
   - "META-INF/"