Default application plan cannot be unset

It is not possible to remove the default selected application plan.

Steps to reproduce:
1. Create a product, and an application plan for it
2. Open the application plans list and see that No plan selected is shown for the Default plan
3. in the drop-down select an application plan and click Change plan
4. Select No plan selected again, and click Change plan

Expected behavior:
The default plan is unset, and No plan selected is shown for the Default plan

Actual behavior:
The following message is shown on the blank screen:

{"status":"Not found"}

The URL path is /apiconfig/services/ID/application_plans/masterize

This is probably related to the Typescript refactor: https://github.com/3scale/porta/pull/3054/files