Null values from yaml should be stored as empty string

When building a flattened map, the YamlProcessor from
Spring Framework, converts a null value to an empty string.
We want the null value to also keep track of its origin,
which is why this commit creates an `OriginTrackedValue`
for an empty string if the original value is null.

Fixes gh-10656

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

@@ -108,7 +108,11 @@ class OriginTrackedYamlLoader extends YamlProcessor {
 
 		private Object constructTrackedObject(Node node, Object value) {
 			Origin origin = getOrigin(node);
-			return OriginTrackedValue.of(value, origin);
+			return OriginTrackedValue.of(getValue(value), origin);
+		}
+
+		private Object getValue(Object value) {
+			return (value != null ? value : "");
 		}
 
 		private Origin getOrigin(Node node) {

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

@@ -114,6 +114,16 @@ public class OriginTrackedYamlLoaderTests {
 		assertThat(getLocation(bar2)).isEqualTo("26:19");
 	}
 
+	@Test
+	public void processEmptyAndNullValues() throws Exception {
+		OriginTrackedValue empty = getValue("empty");
+		OriginTrackedValue nullValue = getValue("null-value");
+		assertThat(empty.getValue()).isEqualTo("");
+		assertThat(getLocation(empty)).isEqualTo("27:8");
+		assertThat(nullValue.getValue()).isEqualTo("");
+		assertThat(getLocation(nullValue)).isEqualTo("28:13");
+	}
+
 	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-yaml.yml

@@ -24,6 +24,8 @@ example:
         bar:
           - bar1: baz
           - bar2: bling
+empty: ""
+null-value: null
 ---
 
 spring: