httppeer: avoid another bad reference before assignment warning
This wasn't a problem, because `b''` from the `AttributeError` handler is in
`bundle2.bundletypes`, so the following loop and conditional always run at least
once. But PyCharm can't figure that out on its own, and it took a little
exploring to figure out it wasn't a problem. The usage in `bundle2.writebundle`
is to look it up in the map of bundle types, so it will break in a more obvious
way in the unlikely event that the empty string is removed from the map in the
future.
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors