feat: 空保护 & update export spec

lib/tallty_import_export/export_payload/cell.rb

@@ -42,7 +42,7 @@ class TalltyImportExport::ExportPayload::Cell
   end
 
   def divise
-    if @header.children.present?
+    if @header.children.present? && @value.value
       new_cells = @header.children.map do |child_header|
         if (child_header.children.present?)
           val = @value.value[child_header.key.to_sym]

lib/tallty_import_export/export_payload/cell_column.rb

@@ -9,7 +9,7 @@ class TalltyImportExport::ExportPayload::CellColumn
     @cell_clusters = []
     @value_handler = value_handler
 
-    if header.children
+    if header.children && @value.value
       @cell_clusters = (@value.value[header.key.to_sym] || []).map do |val|
         TalltyImportExport::ExportPayload::Cell.new(
           @header,
@@ -46,7 +46,7 @@ class TalltyImportExport::ExportPayload::CellColumn
 
     depth_to_cells_mapping = @flatten_cells.group_by { |cell| [cell.header.seq] }
 
-    @tallest_cell_cluster = depth_to_cells_mapping.values.max { |a, b| a.count <=> b.count }
+    @tallest_cell_cluster = depth_to_cells_mapping.values.max { |a, b| a.count <=> b.count } || []
 
     self
   end

spec/export_spec.rb

@@ -82,7 +82,7 @@ RSpec.describe TalltyImportExport::Importable do
 
   it 'export' do
     # TalltyImportExport::ExportForm.new.export_workbook nil, [@payload], { headers: @header_h[:items] }
-    TalltyImportExport::ExportForm.new(nil).export_xlsx [@payload, @payload], { headers: @header_h[:items] }
+    TalltyImportExport::ExportForm.new(nil).export_xlsx [@payload, nil, @payload], { headers: @header_h[:items] }
   end
 
 end