raw_signed_request sometimes returns empty string which is causing NoMethodError: undefined method `size' for nil:NilClass

This is somehow related to this bug:
https://github.com/mkdynamic/omniauth-facebook/issues/84

The `raw_signed_request && parse_signed_request(raw_signed_request)` will not call `parse_signed_request` if `raw_signed_request` is `nil`, but not if it's an empty string, which is not good either. **Sometimes** I am receiving an empty cookie from fb sdk and gem is trying to parse it. This should fix the bug

There should be a check wether `raw_signed_request` is not blank