get0() dates back from the days where Seastar futures carried tuples, and
get0() was a way to get the first (and usually only) element. Now
it's a distraction, and Seastar is likely to deprecate and remove it.
Replace with seastar::future::get(), which does the same thing.
The previous commit has fixed 5 bugs of the same type - incorrectly
passing the default nullptr to one of the changed functions. At
least some of these bugs wouldn't appear if there was no default
value. It's much harder to make this kind of a bug if you have to
write "nullptr". It's also much easier to detect it in review.
Moreover, these default values are rarely used outside tests.
Keeping them is just not worth the time spent on debugging.
