Fix belongs_to polymorphic associations with "empty string" type

[santir489]

Hi folks!
I have encountered the following issue while using this gem.

Issue:
When a model has a belong_to, polymorphic and optional association, and the model has <polymorphic_name>_type = '' set, a NameError exception is raised when saving the model.

Example

class Comment < ActiveRecord::Base
  belongs_to :post, polymorphic: true, optional: true

  has_paper_trail
end

When executing the following line, it fails with NameError (wrong constant name )

Comment.new(post_type: '').save!

This error raises in these lines of code

foreign_type = @record.send(assoc.foreign_type). # returns '' (empty string)
# and then
foreign_type.constantize # raises the error

You can use this script to reproduce the issue.

IMO having polymorphic_type ='' is allowed/supported by Rails, and this gem should not break when trying to versioning an association with this characteristic.