Fix detection of forward() usage when called without parameter

This would cause stateless replies to no be forwared even though the
forward function was used.

(cherry picked from commit 8aab9f2)

Author: rvlad-patrascu

core_cmds.c

@@ -124,7 +124,7 @@ static int w_is_myself(struct sip_msg *msg, str *host, int *port);
 
 static cmd_export_t core_cmds[]={
 	{"forward", (cmd_function)w_forward, {
-		{CMD_PARAM_STR|CMD_PARAM_OPT,
+		{CMD_PARAM_STR|CMD_PARAM_OPT|CMD_PARAM_FIX_NULL,
 			fixup_forward_dest, fixup_free_destination}, {0,0,0}},
 		ALL_ROUTES},
 	{"send", (cmd_function)w_send, {