fix: prevent fetching virtual childred for Composite

[mcollovati]

Description

The Composite class overrides getChildren() to return a stream with the encapsulated component, but also getElement() returning the Element of the wrapped component. The latter causes the virtual children to be fetched as Composite direct children, thus duplicating any virtual children the encapsulated component might have.

Fixes #1738

Type of change

• [x] Bugfix
• [ ] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [ ] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.