Allow fallback to LegacyKeyGenerator when secret_key_base is not set

Let devs upgrading from older Rails take advantage of the LegacyKeyGenerator without needing a monkeypatch to use a blank secret_key_base

This behavior may have been removed by accident.  See https://github.com/rails/rails/commit/484ff7d29e7a27216219cf5460bf675c38abf522


e.g.

```ruby
Rails::Application.class_eval do
  # the key_generator will then use ActiveSupport::LegacyKeyGenerator.new(config.secret_token)
  fail "I'm sorry, Dave, there's no :validate_secret_key_config!" unless instance_method(:validate_secret_key_config!)
  def validate_secret_key_config! #:nodoc:
    config.secret_token = secrets.secret_token
    if config.secret_token.blank?
      raise "Missing `secret_token` for '#{Rails.env}' environment, set this value in `config/secrets.yml`"
    end 
  end 
end
```
@schneems This is more correct, to use secrets.secret_token instead of config.secret_token, no?  Otherwise they might diverge.  (If so, I should probably update the lib, tests, and docs that only use config.secret_token, as well