Rect utility returns wrong sizes in case of sequenced range

[mlewand]

📝 Provide detailed reproduction steps (if any)

While working on #7705 I found out that there are cases when balloon panel being mispositioned, due to the fact that the Rect utility cares only for the first range's client rectangle (while it can contain multiple).

Example repro case:

1. Open https://ckeditor.com/docs/ckeditor5/19.0.0/features/link.html
2. Focus the first editor.
3. Make the following selection The [capital] city
4. Bold it.
5. Make a wider selection, e.g. The [capital city of Malta is the top destination this summer.]
6. Use the link button to open link balloon.

✔️ Expected result

The balloon points to the middle of selection.

❌ Actual result

The balloon point to the beginning of the selection.

Reason is that it contains multiple boxes, the first of which (on chrome from my testing is often) 0px width, thus it points to the very beginning.

📃 Other details

• Browser: Any
• OS: Any

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