Ensure empty plugin settings are saved correctly

[michalkleiner]

Description:

When an array setting was empty, it didn't get sent to the API for processing. We are using a custom empty value to signal that and the API to handle that here.

For Matomo 6 we may consider using json_encoded array instead though that would be a breaking change for now.

You can check that the integration test fails without the JS and PHP changes, and the original issue can also be reproduced by trying to unselect and save all the browsers in the ExampleSettingsPlugin.

Fixes #22877
Ref. DEV-18796

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed

[michalkleiner]

This should actually target 5.2.x-dev. When it's available I can rebase that.

[michalkleiner]

I probably confused GH by the force-push and target branch change, so triggered the tests manually here — https://github.com/matomo-org/matomo/actions/runs/12415416589

[michalkleiner]

Force-pushed to re-trigger the tests.

[caddoo]

Tested and works ✅