mkvmerge: fix decision whether or not to write BlockAdditionMapping

BlockAddIDType is mandatory, all other elements optional. Therefore
require the ID type to be set but don't care about the others.

Part of the implementation of #2887.

src/merge/block_addition_mapping.h

@@ -19,6 +19,6 @@ struct block_addition_mapping_t {
   memory_cptr id_extra_data;
 
   bool is_valid() const {
-    return id_type || id_value || (id_extra_data && id_extra_data->get_size());
+    return !!id_type;
   }
 };