Unmarshaller more resiliant and clearer error if uuid header is missing

[jpradass]

Motivation / Background

I was having issues testing the application using default marshaller and setting the whole thing up as a subscriber listening to a topic and trying to send messages using redis-cli by hand. The thing is that I wasn't sending any uuid header and there wasn't an explicit error in the code to let me know what was happening so I added because I think it is for a better understanding to everyone who try this application that way.

Details

Inside marshaller.go archive, the one that is used as a DefaultMarshaller, inside Unmarshall function, there was an explicit conversion of an interface type variable to a string for an UUID header value and also the payload of the message. I know the marshaller is really basic and it is advisable to use a custom one but I think is clearer for testing purposes if it is added a little if as a check to see if UUID header has been sent in order to warn the user if not and to give "payload" a base value in case it is empty because it will panic otherwise because of a forced nil conversion into a string value.

Alternative approaches considered (if applicable)

Checklist

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[minghsu0107]

The additional check looks good to me, thank you!