Ensure empty layers are written to the index file

Update the `LayersIndex` class to ensure that layers that do not contain
content are still written.

Closes gh-20858

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/java/org/springframework/boot/loader/tools/LayersIndexTests.java

@@ -75,7 +75,7 @@ class LayersIndexTests {
 	}
 
 	@Test
-	void writeToWhenLayerNotUsedSkipsLayer() {
+	void writeToWhenLayerNotUsedDoesNotSkipLayer() {
 		LayersIndex index = new LayersIndex(LAYER_A, LAYER_B, LAYER_C);
 		index.add(LAYER_A, "a1");
 		index.add(LAYER_A, "a2");

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWhenAllFilesInFolderAreInSameLayerUsesFolder.txt

@@ -2,3 +2,4 @@
   - "a1/"
 - "b":
   - "a2/"
+- "c":

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/resources/org/springframework/boot/loader/tools/LayersIndexTests-writeToWhenLayerNotUsedDoesNotSkipLayer.txt

@@ -1,6 +1,7 @@
 - "a":
   - "a1"
   - "a2"
+- "b":
 - "c":
   - "c1"
   - "c2"