Fix another empty nbtree index SSI race.

Fix another empty nbtree index SSI race.

Commit f9b7fc65 fixed a race when predicate-locking completely empty
btrees: without a buffer lock held, a matching key could be inserted
between _bt_search and the PredicateLockRelation call, so the scan would
miss concurrently inserted tuples while the writer wouldn't see the
reader's predicate lock.  That commit only fixed _bt_first's _bt_search
path, though.  Scans without useful insertion scan keys return early
from _bt_first via _bt_endpoint, which still didn't recheck if the
relation was empty.

To fix, add handling to _bt_endpoint that is analogous to the handling
added to _bt_search by commit f9b7fc65.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkNoTn3yXY0iGkSuavJ+sL8EROf+kitW+_2v2tJVWuKmA@mail.gmail.com
Backpatch-through: 14