cmdutils: show options in show_help_children only if the context has options

Avoid to print a pointless name of the context followed by an empty list.

cmdutils.c

@@ -168,8 +168,10 @@ void show_help_options(const OptionDef *options, const char *msg, int mask,
 void show_help_children(const AVClass *class, int flags)
 {
     const AVClass *child = NULL;
-    av_opt_show2(&class, NULL, flags, 0);
-    printf("\n");
+    if (class->option) {
+        av_opt_show2(&class, NULL, flags, 0);
+        printf("\n");
+    }
 
     while (child = av_opt_child_class_next(class, child))
         show_help_children(child, flags);