Filter empty YAML documents

Update `OriginTrackedYamlLoader` so that empty documents are filtered
from the result. Prior to this commit, our origin wrapper would confuse
the YAML processor and cause empty documents to be included in the Map
with a key of "document" and no value.

Closes gh-22493

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

@@ -19,6 +19,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
@@ -82,6 +83,15 @@ List<Map<String, Object>> load() {
 	 */
 	private class OriginTrackingConstructor extends SafeConstructor {
 
+		@Override
+		public Object getData() throws NoSuchElementException {
+			Object data = super.getData();
+			if (data instanceof CharSequence && ((CharSequence) data).length() == 0) {
+				return null;
+			}
+			return data;
+		}
+
 		@Override
 		protected Object constructObject(Node node) {
 			if (node instanceof ScalarNode) {

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

@@ -128,6 +128,13 @@ void unsupportedType() throws Exception {
 		assertThatExceptionOfType(ConstructorException.class).isThrownBy(this.loader::load);
 	}
 
+	@Test
+	void emptyDocumentes() {
+		this.loader = new OriginTrackedYamlLoader(new ClassPathResource("test-empty-yaml.yml", getClass()));
+		List<Map<String, Object>> loaded = this.loader.load();
+		assertThat(loaded).isEmpty();
+	}
+
 	private OriginTrackedValue getValue(String name) {
 		if (this.result == null) {
 			this.result = this.loader.load();

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

@@ -0,0 +1,5 @@
+---
+---
+
+---
+---