Blank Login In sp_who2 Result
I suggest you stop using sp_who2, there are better ways to look at that information.  Even you do not want to use third party script you are better off joining following dmvs and get what you need.

sys.dm_exec_sessions 
sys.dm_exec_requests
sys.dm_exec_sql_text
sys.dm_exec_query_plan
sys.dm_os_tasks

I use sp_whoisactive by Adam Machanic and will not try to write anything by myself.

To answer your question session id 428 is running in parallel meaning multiple threads and you can see it using below query.  Login name is showing only for exec_context_id =0 which is the parent thread.

SELECT er.session_id, 
       er.status, 
       er.command, 
       er.blocking_session_id, 
       er.wait_type, 
       th.os_thread_id,
       os.cpu_id,
       ot.exec_context_id, 
       ot.task_state, 
       st.text 
FROM   sys.dm_os_threads th 
inner join sys.dm_os_tasks ot on th.worker_address = ot.worker_address 
inner join sys.dm_exec_requests er on ot.session_id = er.session_id 
inner join sys.dm_os_schedulers os on th.scheduler_address = os.scheduler_address 
       CROSS apply sys.Dm_exec_sql_text(er.sql_handle) st 
WHERE  er.session_id IN (SELECT session_id 
                         FROM   sys.dm_os_tasks 
                         GROUP  BY session_id 
                         HAVING Count(exec_context_id) &gt; 1
                         )