ITS#9015 syncprov: fix for zero-length suffix

If the "" glue entry exists and lacks a contextCSN, must perform
an additional search to be sure the DB is otherwise empty. If so,
skip creating the contextCSN.
3 jobs for its9015b in 32 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1951
build-gnutls-mit

00:28:50

passed #1949
build-no-threads-no-slapd

00:03:47

passed #1950
build-openssl-heimdal

00:30:24