paypal: fix dereference error in paypal_express_response

Calling PaypalExpressResponse#address when the response didn't contain
any address information was causing a dereference error:

  NoMethodError: undefined method `[]' for nil:NilClass

Testing done: verified the dereference error doesn't happen
by issuing a SetExpressCheckout API Operation with an invalid
security header.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>