The script example with MAPPING LOAD creates a new field in Sessions called Full_Name that will contain either:
the concatenation of Vorname + Nachname if Vorname is Non-NULL, or
the value of [Authenticated user] if Vorname is empty or NULL.
This is all done by the call to applymap(). The mapping table will not have any entries for users with a missing Vorname. Therefore the lookup will fail and applymap() will resort to the value of the third parameter.
So in your chart, just use Full_Name and everything will be fine