Removes empty string from ActiveModel::MassAssignmentSecurity::WhiteList set

Given:
```ruby
# config/application.rb
config.active_record.whitelist_attributes = true

# app/models/user.rb
class User < ActiveRecord::Base
  attr_accessible :name
end
```
As of today for 3-2-stable:
```ruby
User.accessible_attributes # => #<ActiveModel::MassAssignmentSecurity::WhiteList: { "", "name" }>
```
With the proposed change:
```ruby
User.accessible_attributes # => #<ActiveModel::MassAssignmentSecurity::WhiteList: { "name" }>
```