fix: 修复 CellColumn 对于可拆分的判断

lib/tallty_import_export/export_payload/cell_column.rb

@@ -10,7 +10,7 @@ class TalltyImportExport::ExportPayload::CellColumn
     @value_handler = value_handler
 
     val = @value_handler.call(@value.value, @header.as_json.symbolize_keys, keep_array: true)
-    if header.children && val.is_a?(Array)
+    if (header.children&.length || 0) > 0 && val.is_a?(Array)
       @cell_clusters = val.map do |val|
         TalltyImportExport::ExportPayload::Cell.new(
           @header,