[Shipping] Removing tracking code requirement

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #1015
| License       | MIT
| Doc PR        | 

Removing NotBlank constraint on shipment tracking code form. This allows shipments to be shipped without giving a tracking code, as some couriers/postal services don't have tracking codes.

As previously mentioned in issue #1015