rustdoc: Make going back in browser history work after typing a search term
auto merge of #13830 : noamraph/rust/doc-browser-history, r=kballard

Currently, in both chrome and firefox, if I type something in the search box in the reference docs I get immediately the search results. That's great. However, if I want to go back to the doc I was reading and try to press the back button, I am immediately forwarded again to the search results. This is caused by the fact that the search term is (deliberately) left in the search box, and the search() function is called as if I typed the search term again.
I disabled calling the search() function if there's no search term in the URL, and now it seems to work fine.

I hope I'm sending the pull request correctly - I'm not really used to git and github.