LU-14422 utils: check empty stripe-count/offset setting

LU-14422 utils: check empty stripe-count/offset setting

Prohibit "" stripe count and stripe offset setting in lfs setstripe.

Test-Parameters: trivial
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I4d0cc2354c4249a31222e0f90de39e16ec4694a5
Reviewed-on: https://review.whamcloud.com/41793
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>