LU-14711 osc: Do not attempt sending empty pages

LU-14711 osc: Do not attempt sending empty pages

Do not crash if trying to send a lock-prolonging emtpy read
to an old server, if the server does not support short reads.
Otherwise the client crashes when access the NULL page.

Test-Parameters: trivial
Fixes: 564070343ac4 ("LU-14711 osc: Notify server if cache discard takes a long time")
Change-Id: Icae7bf3ef16c45d33894b3c5fbac15b1a98c39d9
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44654
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>