allow null value for attendee phone number (#5132)

Co-authored-by: CarinaWolli <wollencarina@gmail.com>
Co-authored-by: Alex van Andel <me@alexvanandel.com>

apps/web/components/booking/pages/BookingPage.tsx

@@ -262,7 +262,8 @@ const BookingPage = ({
       phone: z
         .string()
         .refine((val) => isValidPhoneNumber(val))
-        .optional(),
+        .optional()
+        .nullable(),
       smsReminderNumber: z
         .string()
         .refine((val) => isValidPhoneNumber(val))