fix .zero? error in id_attribute check

[NGeb1]

I got this error when searching:

[6adb7d18-ed52-428b-ad9c-8c602234****] NoMethodError (undefined method `zero?' for "25496":String

          next if dmsf_attrs.length.zero? || id_attribute.zero?
                                                     ^^^^^^):
[6adb7d18-ed52-428b-ad9c-8c602234****]
[6adb7d18-ed52-428b-ad9c-8c602234****] plugins/redmine_dmsf/app/models/dmsf_file.rb:425:in `block in search'

so I added ".to_i" before the .zero? to fix the error.

[picman]

Thank you!