unsafe fn searcher_kind_empty(
    _searcher: &Searcher,
    _prestate: &mut PrefilterState,
    _haystack: &[u8],
    _needle: &[u8],
) -> Option<usize> {
    Some(0)
}
Reads from the `empty` field of `SearcherKind` to handle the case of
searching for the empty needle. Works on all platforms.

# Safety

Callers must ensure that the `searcher.kind.empty` union field is set.
src/memmem/searcher.rs