Please look at https://quarry.wmflabs.org/query/20617 for some examples of logs where log_user is there but log_user_text is null (in this case, it should have been 'ماني').
I am not sure if this is caused by the Labs views, or if the actual data in the DB is also having log_user_text as NULL for these rows. Some (but not all) of these are logs that are later revdel'ed so that may have to do with this. For now, I am tagging it both for Clouds Services and for WMF Site Requests, but it should be narrowed down to only one of those two by someone who has access to the DB tables.