Bind Optional value object parameters as empty rather than null

Update `ValueObjectBinder` so that `Optional` parameters are bound as
`Option.empty()` rather than `null`.

See gh-49152

Signed-off-by: Ondřej Světlík <16480390+osvetlik@users.noreply.github.com>

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

@@ -105,7 +105,13 @@ final class BindConverter {
 				new ResolvableTypeDescriptor(targetType, targetAnnotations));
 	}
 
-	private @Nullable Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
+	@SuppressWarnings("unchecked")
+	<T> @Nullable T convertNullValue(ResolvableType targetType, Annotation... targetAnnotations) {
+		return (T) convert(null, null, new ResolvableTypeDescriptor(targetType, targetAnnotations));
+	}
+
+	private @Nullable Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType,
+			TypeDescriptor targetType) {
 		ConversionException failure = null;
 		for (ConversionService delegate : this.delegates) {
 			try {

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

@@ -132,7 +132,12 @@ class ValueObjectBinder implements DataObjectBinder {
 				return convertDefaultValue(context.getConverter(), defaultValue, type, annotations);
 			}
 		}
-		return null;
+		return convertNullValue(context, type);
+	}
+
+	private <T> @Nullable T convertNullValue(Binder.Context context, ResolvableType type, Annotation... annotations) {
+		BindConverter converter = context.getConverter();
+		return converter.convertNullValue(type, annotations);
 	}
 
 	private <T> @Nullable T convertDefaultValue(BindConverter converter, String[] defaultValue, ResolvableType type,

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

@@ -326,6 +326,25 @@ class ValueObjectBinderTests {
 		assertThat(bound.getOptionalValue()).isEmpty();
 	}
 
+	@Test
+	void bindWhenOptionalParameterWithNoValueForPrimitiveShouldReturnEmptyInstance() {
+		RecordPropertiesWithOptional bound = this.binder.bindOrCreate("foo",
+				Bindable.of(RecordPropertiesWithOptional.class));
+		assertThat(bound.property1()).isEmpty();
+	}
+
+	@Test
+	void bindWhenOptionalParameterInNestedWithNoValueForPrimitiveShouldReturnEmptyInstance() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.nested.property2", "str");
+		this.sources.add(source);
+		RecordPropertiesWithOptionalInNestedRecord bound = this.binder.bindOrCreate("foo",
+				Bindable.of(RecordPropertiesWithOptionalInNestedRecord.class));
+		assertThat(bound.nested()).isNotNull();
+		assertThat(bound.nested().property1()).isNotNull().isEmpty();
+		assertThat(bound.nested().property2()).isEqualTo("str");
+	}
+
 	@Test
 	void bindWhenEnumParameterWithEmptyDefaultValueShouldThrowException() {
 		assertThatExceptionOfType(BindException.class)
@@ -902,6 +921,14 @@ class ValueObjectBinderTests {
 			@DefaultValue("default-value-2") String property2) {
 	}
 
+	record RecordPropertiesWithOptional(Optional<String> property1) {
+	}
+
+	record RecordPropertiesWithOptionalInNestedRecord(NestedRecord nested) {
+		record NestedRecord(Optional<String> property1, String property2) {
+		}
+	}
+
 	static class NonExtractableParameterName {
 
 		private @Nullable String value;