Ensure initially selected site is correctly emitted #18401

Description:

When the site selector is used as a form field, the initially selected site might currently not be emitted correctly.
This can be seen e.g. when trying to add a new site config in search performance plugin.
The site selector initially shows a site, but the form field it is referenced to (using ng-model="form.site.id") won't have an initial value.

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

@sgiehl this should be fixed in the vue-form-field branch, can you check? The selectedSite state is removed so this fix wouldn't apply there.

will check that

@diosmosis yes. works with the changes of the other PR. closing this one then.