fix(@vtmn/react): remove default label value in `VtmnDropdown`

[0x2A-git]

Changes description

This P.R aims to solve #1399 .

VtmnDropdown's label has been set to undefined by default to match the component's specification :

Context

Checklist

• [X] Make sure you are requesting to pull a topic/feature/bugfix branch. Please, don't request directly from your main!
• [X] Check commits & PR names matches our requested structure. It must follow the https://www.conventionalcommits.org pattern.
• [X] Check your code additions will fail neither code linting checks.
• [X] I have reviewed the submitted code.
• [X] I have tested on related showcases.
• [X] If it includes design changes, please ask for a review design-system-core-team-design GitHub team.

Does this introduce a breaking change?

• No

Other information