fix: 解决未关联对象的问题

lib/tallty_import_export/export.rb

@@ -94,7 +94,9 @@ module TalltyImportExport
         association_records.each do |association_record|
           row = []
           headers.each_with_index do |header, col_index|
-            records = @each_method.present? ? try_method(association_record, @each_method) : [association_record]
+            records = @each_method.present? ?
+              (try_method(association_record, @each_method) || [nil]) :
+              [association_record]
             records.each do |record|
               index += 1
               _data = header[:source] ?