Extend default handling from undefined-only to null/empty argument objects

[tcompa]

Given #538, we have case A (arguments object is "empty" and fractal-web fills the defaults) and case B (arguments objects has values, and fractal-web ignore defaults).

Right now, case A actually requires arguments to be "undefined". We should extend it to cases where the object exists but is either null or {}.

This will also make the sandbox behavior (where undefined is sometimes found) more similar to the webclient (where the backend always sends something for the arguments property).