Add origin support for empty YAML list and map

Update `OriginTrackedYamlLoader` to better deal with empty maps
and collections.

See gh-21704

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/OriginTrackedYamlLoader.java

@@ -30,6 +30,7 @@ import org.yaml.snakeyaml.constructor.BaseConstructor;
 import org.yaml.snakeyaml.constructor.Constructor;
 import org.yaml.snakeyaml.constructor.SafeConstructor;
 import org.yaml.snakeyaml.error.Mark;
+import org.yaml.snakeyaml.nodes.CollectionNode;
 import org.yaml.snakeyaml.nodes.MappingNode;
 import org.yaml.snakeyaml.nodes.Node;
 import org.yaml.snakeyaml.nodes.NodeTuple;
@@ -109,6 +110,9 @@ class OriginTrackedYamlLoader extends YamlProcessor {
 					return constructTrackedObject(node, super.constructObject(node));
 				}
 			}
+			else if (node instanceof CollectionNode && ((CollectionNode<?>) node).getValue().isEmpty()) {
+				return constructTrackedObject(node, super.constructObject(node));
+			}
 			else if (node instanceof MappingNode) {
 				replaceMappingNodeKeys((MappingNode) node);
 			}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/env/OriginTrackedYamlLoaderTests.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.env;
 
 import java.nio.charset.StandardCharsets;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
@@ -120,6 +121,14 @@ class OriginTrackedYamlLoaderTests {
 		assertThat(getLocation(nullValue)).isEqualTo("28:13");
 	}
 
+	@Test
+	void processEmptyListAndMap() {
+		OriginTrackedValue emptymap = getValue("emptymap");
+		OriginTrackedValue emptylist = getValue("emptylist");
+		assertThat(emptymap.getValue()).isEqualTo(Collections.EMPTY_MAP);
+		assertThat(emptylist.getValue()).isEqualTo(Collections.EMPTY_LIST);
+	}
+
 	@Test
 	void unsupportedType() throws Exception {
 		String yaml = "value: !!java.net.URL [!!java.lang.String [!!java.lang.StringBuilder [\"http://localhost:9000/\"]]]";

spring-boot-project/spring-boot/src/test/resources/org/springframework/boot/env/test-yaml.yml

@@ -26,6 +26,8 @@ example:
           - bar2: bling
 empty: ""
 null-value: null
+emptylist: []
+emptymap: {}
 ---
 
 spring: