LU-11947 scripts: handle ZFS targets in Lustre RA

LU-11947 scripts: handle ZFS targets in Lustre RA

Fixes a regression introduced in LU-11461
This handles the case of realpath of target being an empty string.

Fixes: c36d70272541 ("LU-11461 scripts: Support symlink target")
Test-Parameters: trivial
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: I1bcb85908019e968ac0d69e437db217594a6565e
Reviewed-on: https://review.whamcloud.com/34217
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>