Revert "all: Don't set AVClass.item_name to its default value"

Some callers assume that item_name is always set, so this may be
considered an API break.

This reverts commit 0c6203c97a.

libavfilter/avfiltergraph.c

@@ -61,6 +61,7 @@ static const AVOption filtergraph_options[] = {
 
 static const AVClass filtergraph_class = {
     .class_name = "AVFilterGraph",
+    .item_name  = av_default_item_name,
     .version    = LIBAVUTIL_VERSION_INT,
     .option     = filtergraph_options,
     .category   = AV_CLASS_CATEGORY_FILTER,