Fix crate filter search reset
I found a fun bug when using rustdoc recently: I made a search, cut the search input content, changed the crate filter, pasted back the input content. To my surprise, the crate filter wasn't applied. It's because that our search input was empty when receiving the `<select>` "onchange" event. To fix this issue, I reset the `currentResults` variable to `null`.
It's using the first commit from #86542 so it needs to wait for it before getting merged.
r? `@jsha`
b6f3cb9502