LU-6027 mdt: Allow user EAs with empty values

LU-6027 mdt: Allow user EAs with empty values

Setting a user EA with an empty value is a valid case, according to
attr(5) and some experiments with ext4.  Doing so with Lustre
currently results in what appears to be a no-op---the EA name won't be
added or updated (if an EA with the same name already existed).

Change-Id: Ic8950963baeceada99c4607631ecd2a6510ae3ed
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/12900
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>