gitweb: make "No commits" in project list gray, not bold green

A missing return statement in git_get_last_activity made gitweb think
a project with no commits was in age class "age0", so the "No commits"
appeared in bold green, which was ridiculous.  I added the return so
those projects get "noage" and added a block to gitweb.css to format
the "No commits" text gray.

Signed-off-by: Matt McCutchen <hashproduct@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -1526,6 +1526,7 @@ sub git_get_last_activity {
 		my $age = time - $timestamp;
 		return ($age, age_string($age));
 	}
+	return (undef, undef);
 }
 
 sub git_get_references {