Align DBSelect Documentation on `valid` state

Align visible "placeholder" in between Figma and Development. It shouldn't be a selected "Valid" entry, but showing the "placeholder", that should be "valid".

<img width="440" alt="Image" src="https://github.com/user-attachments/assets/5d61bdc6-e8d0-4462-a79b-bfd92445a22a" />

https://design-system.deutschebahn.com/core-web/version/v2.2.5/components/data-input/select/overview