Passing --disable-per-crate-search to rustdoc disables search altogether

[jyn514]

docs.rs ran into this this morning, see rust-lang/docs.rs#497.

This seems to have been caused by #66298, which disabled the search bar by default. The logic to reenable search was a little buggy - getSearchElement() looks for an element with the 'search' id, but that ID is not set when --disable-per-crate-search is enabled.

Steps to replicate:

cat '/// Hello, world!
fn main() {
}' > tmp.rs
rustdoc +nightly -Zunstable-options --disable-per-crate-search tmp.rs
xdg-open doc/tmp/index.html

cc @QuietMisdreavus @GuillaumeGomez

[GuillaumeGomez]

I'll try to take a look tomorrow.