bug(@vtmn/css): enhancement `VtmnSelect` make the props label optional

[Mig-l]

Describe the bug

We need to use the VtmnSelect without the label.
The label props is mandatory

If we pass the label props with an empty string, we still have an HTML label tag that is generated but empty.

Steps to reproduce

https://decathlon.github.io/vitamin-web/@vtmn/showcase-svelte/?path=/story/components-forms-vtmnselect--overview

Expected behavior

We have in production a select without label for the choice of the language that we must switch to Vitamin

We will want to use the VtmnSelect without the label

[stale]

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