Fix for not entering a pin while setting your pin code

In the current codebase, https://github.com/adhearsion/voicemail/blob/develop/lib/voicemail/call_controllers/mailbox_set_pin_controller.rb#L32 will explode with an `undefined method `size' for nil:NilClass` if a user hits the terminator twice without entering any other DTMF.

This fixes that.