initialize xkb_rules.options to a null pointer

Initializing it to an empty string had broken configuring xkbcommon
through the environment (XKB_DEFAULT_OPTIONS).

Fixes: ae313911153b ("initialize rules and xkb_rules")

config.def.h

@@ -41,7 +41,7 @@ static const struct xkb_rule_names xkb_rules = {
 	/* example:
 	.options = "ctrl:nocaps",
 	*/
-	.options = "",
+	.options = NULL,
 };
 
 static const int repeat_rate = 25;