Merged PR 2653: fixed bookings test to include null for verification_request_booking_source

# Description

Fixed bookings test to include null for verification_request_booking_source

# Checklist

- [X] The edited models and dependants run properly with production data.
- [X] The edited models are sufficiently documented.
- [X] The edited models contain PK tests, and I've ran and passed them.
- [X] I have checked for DRY opportunities with other models and docs.
- [X] I've picked the right materialization for the affected models.

# Other

- [ ] Check if a full-refresh is required after this PR is merged.

Related work items: #20231

models/intermediate/core/schema.yaml

@@ -1721,13 +1721,14 @@ models:
           - PMS
           - OSL
           - API/MANUAL
+          - null (bookings without verification request)
         tests:
-          - not_null
           - accepted_values:
                 values:
                   - 'PMS'
                   - 'OSL'
                   - 'API/MANUAL'
+                  - null
 
       - name: id_staging_host_booking
         data_type: bigint