Allow empty search string

This allows to search with an empty search string. If the search string is empty, the ```search``` string does not get added to the url.
It is then possible to search for example for every Tor node within a given AS.

I preferred to allow the empty string instead of introducing some wildcard character like *, since the onionoo API works the way that ```search``` is just treated like all the other filters like ```as``` as far is i understand.

If you don't like the approach, i'd be happy to change it if you give me some advice.