fix(ads): placement control value reset

[miguelpeixe]

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Changes proposed in this Pull Request:

NPPM-2441

The current strategy to infer and set a default provider in the PlacementControl component doesn't play well when the initial state is an invalid option. The UI will show the default value, but never trigger the change, which will mislead the user.

This PR fixes this behavior by removing the default provider.

How to test the changes in this Pull Request:

1. While on trunk, make sure you have GAM and Broadstreet configured and enabled
2. Set some global placements to use GAM
3. Deactivate the GAM provider
4. Edit the placement and confirm that it automatically shows Broadstreet as the provider
5. Confirm that saving the placement doesn't change its value in the database
6. Checkout this branch, refresh the page and edit the placement again
7. Confirm it shows "Select a provider"
8. Set Broadstreet, choose an ad unit, and save
9. Confirm the values are updated in the database

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[github-actions]

Hey @miguelpeixe, good job getting this PR merged! 🎉

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