fix: validate date-time-picker when required is set to false (#7908) (#7914)

Co-authored-by: Serhii Kulykov <iamkulykov@gmail.com>

Author: vaadin-bot

packages/date-time-picker/src/vaadin-date-time-picker.js

@@ -715,6 +715,12 @@ class DateTimePicker extends FieldMixin(DisabledMixin(FocusMixin(ThemableMixin(E
     if (timePicker) {
       timePicker.required = required;
     }
+
+    if (this.__oldRequired && !required) {
+      this.validate();
+    }
+
+    this.__oldRequired = required;
   }
 
   /** @private */

packages/date-time-picker/test/validation.test.js

@@ -200,6 +200,11 @@ const fixtures = {
         dateTimePicker.validate();
         expect(dateTimePicker.invalid).to.be.true;
       });
+
+      it('should validate when setting required to false', () => {
+        dateTimePicker.required = false;
+        expect(validateSpy).to.be.calledOnce;
+      });
     });
 
     describe('document losing focus', () => {