All blank or empty parameters can be undefined for the search, which
means they won't be included in the API call URL in silly ways like e.g.
/questions?searchString=&tags=&examBoards=&subjects=physics&...
in every single request.

The API treats these empty string values identically to the null case,
except "statuses", where the empty string means "don't filter by status
but _do_ augment results with attempt data". So for the question finder,
this argument must never be undefined but may well meaningfully be "".

…earch-url

Exclude unnecessary URL params on question search

It happens that ElasticSearch does treat the spaces in a way that means
the query _happens_ to work, but we should not be relying on this!

(cherry picked from commit 66e12b1)