lavfi/avfiltergraph: avoid to print "(null)" in the scale args

Fix parsing.

libavfilter/avfiltergraph.c

@@ -358,7 +358,11 @@ static int query_formats(AVFilterGraph *graph, AVClass *log_ctx)
 
                     snprintf(inst_name, sizeof(inst_name), "auto-inserted scaler %d",
                              scaler_count++);
-                    snprintf(scale_args, sizeof(scale_args), "0:0:%s", graph->scale_sws_opts);
+                    if (graph->scale_sws_opts)
+                        snprintf(scale_args, sizeof(scale_args), "0:0:%s", graph->scale_sws_opts);
+                    else
+                        snprintf(scale_args, sizeof(scale_args), "0:0");
+
                     if ((ret = avfilter_graph_create_filter(&convert, filter,
                                                             inst_name, scale_args, NULL,
                                                             graph)) < 0)