feat: export header children

lib/tallty_import_export/attr/export_header.rb

@@ -22,7 +22,7 @@ module TalltyImportExport
       def calcute_flatten_value _depth, _parent_path
         @depth = _depth
         @parent_path = _parent_path
-        if children
+        if children && children.count > 0
           children.map do |child|
             child.calcute_flatten_value(_depth + 1, [*_parent_path, self])
           end.compact.flatten