auth-filters: do not crash on nil username

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

filters/gentoo-ldap-authentication.lua

@@ -106,7 +106,7 @@ local lualdap = require("lualdap")
 
 function gentoo_ldap_user_groups(username, password)
 	-- Ensure the user is alphanumeric
-	if username:match("%W") then
+	if username == nil or username:match("%W") then
 		return nil
 	end