The pager::state() function returns a valid paging object even
if the pager itself is exhausted. It may also not contain the partition
key, so using it unconditionally was a bug - now, in case there is no
partition key present, paging state will contain an empty partition key.
Fixes #3829
Message-Id: <28401eb21ab8f12645c0a33d9e92ada9de83e96b.1539074813.git.sarna@scylladb.com>
(cherry picked from commit b3685342a6)