Allow empty objects to be loaded from YAML

Update `OriginTrackedYamlLoader` so that empty objects are retained
in the resulting `PropertySource`.

Closes gh-48958

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

@@ -82,7 +82,7 @@ class OriginTrackedYamlLoader extends YamlProcessor {
 
 	List<Map<String, Object>> load() {
 		List<Map<String, Object>> result = new ArrayList<>();
-		process((properties, map) -> result.add(getFlattenedMap(map)));
+		process((properties, map) -> result.add(getFlattenedMap(map, true, "")));
 		return result;
 	}
 

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

@@ -39,6 +39,7 @@ import org.springframework.core.io.Resource;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.entry;
 
 /**
  * Tests for {@link OriginTrackedYamlLoader}.
@@ -154,9 +155,8 @@ class OriginTrackedYamlLoaderTests {
 
 	@Test
 	@WithTestYamlResource
-	void emptyMapsAreDropped() {
-		Object emptyMap = getValue("emptymap");
-		assertThat(emptyMap).isNull();
+	void emptyMapsOrObjectsAreNotDropped() {
+		assertThat(getFirstResult()).contains(entry("emptymap", ""));
 	}
 
 	@Test
@@ -250,10 +250,15 @@ class OriginTrackedYamlLoaderTests {
 
 	@SuppressWarnings("unchecked")
 	private <T> @Nullable T getValue(String name) {
+		return (T) getFirstResult().get(name);
+	}
+
+	private Map<String, Object> getFirstResult() {
 		if (this.result == null) {
 			this.result = this.loader.load();
 		}
-		return (T) this.result.get(0).get(name);
+		Map<String, Object> map = this.result.get(0);
+		return map;
 	}
 
 	private String getLocation(OriginTrackedValue value) {