Default value in Document

I would like to know if it exist a way to put default value in document.
I mean, imagine you have a document containing a lot of string variables and in the flow, at run time, only few of them are set.
In runtime when we look at the document in debug, we see a document with only the few variables set and not all variables.
I’m looking for an algorithm or settings to see all variables in document (even we have null in value or blank).

Before you suggest the idea to initialize all variables with set new value with “” before mapping value, I prefere to say some documents have many variables and I would like to win time. Beside, for some generical flow which accept document in input (without the containes of the document), the algorithm browse list of variable (if not initialize, there is no variable)