Fix wrong value of MObject width/height with empty VGroup

[gvauvert]

Overview: What does this pull request change?

Fixes #4087

When a Mobject contains an empty VGroup, its width or height can be wrong

Motivation and Explanation: Why and how do your changes improve the library?

Bug fix + tests

Links to added or changed documentation pages

No documentation page, bug fix only.

Further Information and Comments

• add test
• fix bug
• fix another test which result depends on the bug

Reviewer Checklist

• [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• [ ] If applicable: newly added functions and classes are tested