Disallow the empty string as an attribute name

Disallow the empty string as an attribute name

Previously, it was possible to have a line like "file.txt =foo" in a
.gitattribute file, after which an invocation like "git check-attr ''
-- file.txt" would succeed.  This patch disallows both constructs.

Please note that any existing .gitattributes file that tries to set an
empty attribute will now trigger the error message "error: : not a
valid attribute name" whereas previously the nonsense was allowed
through.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>