CELGenUserEvent: Fix error message from ast_json_pack

This prevents NULL from being passed into an ast_json_pack call when no
extra information is passed to the application which prevents an error
message about NULL arguments from being generated.
........

Merged revisions 402755 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402756 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_celgenuserevent.c

@@ -77,7 +77,7 @@ static int celgenuserevent_exec(struct ast_channel *chan, const char *data)
 
 	blob = ast_json_pack("{s: s, s: {s: s}}",
 		"event", args.event,
-		"extra", "extra", args.extra);
+		"extra", "extra", S_OR(args.extra, ""));
 	if (!blob) {
 		return res;
 	}