DecodeSignal: Provide conversion data container sooner

DecodeSignal: Provide conversion data container sooner

Before, the converted_data_ container was only created
once we had sample data coming in. This meant that it
wasn't possible to assign a converted signal to a decoder.

With this change, the data container is created even
when there is no data to fill it with, allowing logic_data()
to return a valid result and in turn allowing the user
to assign the signal to a decoder.