glusterfs: allow setxattr of keys with null values.

Disk based file systems allow to get/set extended attribute key-value pairs where
value can be null. Fuse/libgfapi clients must be able to do the same on a
gluster volume.

Change-Id: Ifc11134cc07f1a3ede43f9d027554dcd10b5c930
BUG: 1135514
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/8567
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>