s3:mdssvc: fix slrpc_fetch_attributes() when CNID is not known

s3:mdssvc: fix slrpc_fetch_attributes() when CNID is not known

Samba currenlty fails the whole RPC request, macOS returns returns a nil entry
for the requested CNID:

DALLOC_CTX(#1): {
	sl_array_t(#3): {
		uint64_t: 0x0000
		CNIDs: unkn1: 0xfec, unkn2: 0x6b000020
			DALLOC_CTX(#1): {
				uint64_t: 0xe4bbf314c03b1e
			}
		sl_filemeta_t(#1): {
			sl_array_t(#2): {
				nil
				nil
			}
		}
	}
}

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Aug  8 21:43:14 UTC 2019 on sn-devel-184