[Vue] Make sure default value for modelValue in SiteSelector is reflected in v-model value. #19369

Description:

When null/undefined is used as the v-model value for SiteSelector, SiteSelector sets a default value. But this is not reflected in v-model since the update:modelValue event is not called, leaving the parent component's v-model value still set to undefined/null.

This PR makes sure the default value is set through update:modelValue.

It also adds a new property defaultToFirstSite that allows defaulting the modelValue to the first site in the queried lists. This can't be done by default in case it needs to be possible to unset the value to null. (I'm not sure if that's currently possible, though.)

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