fix(docs): move placeholder to SelectValue attribute in field-select (#11486)

[Rodrigoue9]

Title

fix(docs): move placeholder from items list to SelectValue placeholder attribute (#11486)

Description

• Removes null dummy option { label: "Choose department", value: null } from items in base field-select example.
• Sets placeholder="Choose department" on SelectValue to match radix example and prevent placeholder from appearing as selectable list item.

Closes #11486

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