Log file Warning: "User is empty."

After an import process, you can see in the log file a WARNING message displaying "User is empty.", as this below example:  

[2019-03-13 14:55:48] Verbose ------------------------------------------------------------------
[2019-03-13 14:55:48] Verbose List 'Lists/Memo' - Process Item 'NT0000094E' : [12/34]
[2019-03-13 14:55:48] Information Adding attachments for item
[2019-03-13 14:55:48] Information Setting field values for item
[2019-03-13 14:55:48] Information Processing HTML fields for item
[2019-03-13 14:55:48] WARNING User is empty.
[2019-03-13 14:55:48] WARNING -> Fallback to: 'i:0#.w|cooperteam\sharepointadmin': 
[2019-03-13 14:55:48] WARNING User is empty.
[2019-03-13 14:55:48] WARNING -> Fallback to: 'i:0#.w|cooperteam\sharepointadmin': 
[2019-03-13 14:55:48] Verbose ------------------------------------------------------------------

This corresponds to the search for the user to insert in the "Created by" and "Modified by" fields of SharePoint.

This warning is caused because the XML document (Document_0000094E.xml) doesn't contain a $UpdatedBy tag in the <ITEMS> section, like this below example: