fix: binder to update value on validation success (#23172)

After failed validation for null value
write null to bean when validatio passes.

Fixes vaadin/flow-components#8209

Author: caalador

flow-data/src/main/java/com/vaadin/flow/data/binder/Binder.java

@@ -1391,8 +1391,8 @@ public BindingImpl(BindingBuilderImpl<BEAN, FIELDVALUE, TARGET> builder,
                     && getField() instanceof HasValidator) {
                 HasValidator<FIELDVALUE> hasValidatorField = (HasValidator<FIELDVALUE>) getField();
                 onValidationStatusChange = hasValidatorField
-                        .addValidationStatusChangeListener(
-                                event -> this.validate());
+                        .addValidationStatusChangeListener(event -> getBinder()
+                                .handleFieldValueChange(this));
             }
 
             this.getter = getter;

flow-data/src/test/java/com/vaadin/flow/data/binder/BinderValidationStatusChangeListenerTest.java

@@ -184,4 +184,41 @@ public void fieldWithHasValidatorFullyOverridden_boundFieldGetsUnbind_validation
         Assert.assertEquals(INVALID_DATE_FORMAT, componentErrors.get(field));
     }
 
+    @Test
+    public void fieldWithHasValidator_validationStatusChangesToTrueWithNullValue_beanIsUpdated() {
+        // Setup: bind field to bean with an initial date value
+        var field = new TestHasValidatorDatePicker.DataPickerHasValidatorOverridden();
+        LocalDate initialDate = LocalDate.of(2023, 1, 15);
+        item.setBirthDate(initialDate);
+        binder.bind(field, BIRTH_DATE_PROPERTY);
+        binder.setBean(item);
+
+        // Verify initial state
+        Assert.assertEquals(initialDate, item.getBirthDate());
+        Assert.assertEquals(initialDate, field.getValue());
+
+        // Simulate: user enters invalid input (field keeps null value
+        // internally,
+        // validation fails)
+        // field.setValue(null);
+        field.fireValidationStatusChangeEvent(false);
+
+        // Bean should still have old value since validation failed
+        Assert.assertEquals(1, componentErrors.size());
+        Assert.assertEquals(initialDate, item.getBirthDate());
+
+        // Simulate: user clears the field to null (which is now accepted)
+        // Field value is already null, but validation now passes
+        field.setValue(null);
+        field.fireValidationStatusChangeEvent(true);
+
+        // Error should be cleared
+        Assert.assertEquals(0, componentErrors.size());
+
+        // Bug: Bean should be updated to null, but currently it's not
+        Assert.assertNull(
+                "Bean property should be updated to null when validation passes",
+                item.getBirthDate());
+    }
+
 }

flow-data/src/test/java/com/vaadin/flow/data/binder/testcomponents/TestDatePicker.java

@@ -11,7 +11,8 @@ public class TestDatePicker
     private String label = null;
 
     public TestDatePicker() {
-        super(null, LocalDate::parse, LocalDate::toString);
+        super(null, LocalDate::parse,
+                value -> value != null ? value.toString() : null);
     }
 
     public TestDatePicker(String label) {