Bug: UI should prevent user from creating multiple empty platform selections in Create/Edit registry page

[xingzhang-suse]

When user click the add platform multiple time, too many empty selection can be created.
Then after submitting the registry, it will trigger validation error.

Expected behavior

• Don't allow user to add next platform unless the previous one is valid.
• When submitting registry, not to set the platform filter into the payload, if the last platform is empty.