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 @@ package org.springframework.boot.env;
 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 @@ class OriginTrackedYamlLoader extends YamlProcessor {
 	 */
 	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 @@ class OriginTrackedYamlLoaderTests {
 		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 @@
+---
+---
+ 
+---
+---