nfscl: Fix support for doing Null RPCs

Although the NFS client does not currently perform Null RPCs,
this fix is needed if/when it might do so.
Found during testing of experimental code that uses Null RPCs
to maintain/monitor TCP connections for "nconnect" mounts.

(cherry picked from commit 4adb28c0abfa3b8d076c7249f61a093c6eb9dac2)