Don't put '?' on the end of parameterless URLs.

Don't put '?' on the end of parameterless URLs.

If you call reqwest::Url::parse_with_params and give it an empty
parameter iterator, it still appends '?' to the URL, and then doesn't
put anything after it.

I don't _think_ that actually makes any difference, but it looks ugly.
More to the point, I noticed it while pasting HTTP transcripts to
report a bug in the Mastodon dev setup, and thought I'd better try
again without the spurious '?', in case it did make a difference.