SLAPI fix - no-op when slapi_plugins_used == 0

servers/slapd/acl.c

servers/slapd/add.c

servers/slapd/backend.c

servers/slapd/bind.c

servers/slapd/compare.c

servers/slapd/config.c

servers/slapd/connection.c

servers/slapd/delete.c

servers/slapd/modify.c

servers/slapd/modrdn.c

servers/slapd/operation.c

servers/slapd/proto-slap.h

servers/slapd/result.c

servers/slapd/search.c

servers/slapd/syncrepl.c