CTHUB - exclude empty postal codes from triggering warnings

**Describe the task**
Currently, during dataset validation, missing postal codes generate warnings. We need to update the logic so that empty values in the postal code no longer trigger for warnings.

**Purpose**
This change aligns with the updated requirements discussed with the Katia.

**Acceptance Criteria**
- [ ] Postal code field left empty in the dataset does not trigger a warning

**Additional context**
Reference to the original request where a warning for empty postal codes was initially requested [385](https://github.com/bcgov/cthub/issues/385)