added migration to replace existing file_extension containing nil with ''

db/migrate/20230206203117_migrate_filetype_extensions_not_nil.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class MigrateFiletypeExtensionsNotNil < ActiveRecord::Migration[7.0]
+  def change
+    FileType.all.find_all {|file_type| file_type.file_extension.nil? }.each do |file_type|
+      file_type.update file_extension: ''
+    end
+  end
+end