Return correct error on query of xattr on file with empty xattrs

xfstest 020 detected a problem with cifs xattr handling.  When a file
had an empty xattr list, we returned success (with an empty xattr value)
on query of particular xattrs rather than returning ENODATA.
This patch fixes it so that query of an xattr returns ENODATA when the
xattr list is empty for the file.

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>