[Vue] Correctly handle no group name in GroupedSettings. #18742

Description:

The prefix should not include "." in this case.

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 it won't cause any issues, it's only used directly in vue code, and all the existing code supplies a groupName (so its never empty). Only noticed the issue w/ tag manager where it's not used.