Bind empty strings to empty maps

Align `MapBinder` with `IndexedElementsBinder` to ensure that
empty strings are bound as empty `Map` instances rather than
throwing a `ConverterNotFoundException`.

See gh-50773

Signed-off-by: Ahmed El amraouiyine <ahmed.elamraouiyine@vilavi.fr>

core/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/MapBinder.java

@@ -33,6 +33,7 @@ import org.springframework.boot.context.properties.source.ConfigurationPropertyS
 import org.springframework.boot.context.properties.source.IterableConfigurationPropertySource;
 import org.springframework.core.CollectionFactory;
 import org.springframework.core.ResolvableType;
+import org.springframework.util.ObjectUtils;
 
 /**
  * {@link AggregateBinder} for Maps.
@@ -64,6 +65,9 @@ class MapBinder extends AggregateBinder<Map<Object, Object>> {
 				if (property != null) {
 					getContext().setConfigurationProperty(property);
 					Object result = getContext().getPlaceholdersResolver().resolvePlaceholders(property.getValue());
+					if (ObjectUtils.isEmpty(result)) {
+						return createMap(target);
+					}
 					return getContext().getConverter().convert(result, target);
 				}
 			}

core/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/MapBinderTests.java

@@ -274,6 +274,15 @@ class MapBinderTests {
 		assertThat(result.isBound()).isFalse();
 	}
 
+	@Test
+	void bindToMapWhenEmptyStringShouldReturnEmptyMap() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo", "");
+		this.sources.add(source);
+		Map<String, String> result = this.binder.bind("foo", STRING_STRING_MAP).get();
+		assertThat(result).isEmpty();
+	}
+
 	@Test
 	void bindToMapShouldConvertKey() {
 		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
@@ -510,6 +519,17 @@ class MapBinderTests {
 		assertThat(foo2.getValue()).isEqualTo("three");
 	}
 
+	@Test
+	void nestedMapsWhenEmptyStringShouldReturnEmptyMap() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.value", "one");
+		source.put("foo.foos", "");
+		this.sources.add(source);
+		BindResult<NestableFoo> foo = this.binder.bind("foo", NestableFoo.class);
+		assertThat(foo.get().getValue()).isEqualTo("one");
+		assertThat(foo.get().getFoos()).isEmpty();
+	}
+
 	@Test
 	void bindToMapWithCustomConverter() {
 		DefaultConversionService conversionService = new DefaultConversionService();