[libc++][ranges] Applied `[[nodiscard]]` to `empty_view` (#173215)

`[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue.

- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/ranges
- https://wg21.link/range.empty

Towards #172124

NOKEYCHECK=True
GitOrigin-RevId: 32236eda8257a8b2ebc0f6d326ae645e10000ede