How to show NULL in fields where the condition is not satisfied in SQL

Question

I have this Input table (p):

EmployeeID  EventId     EntityId    EnterDate
11111       100         1           2:54:11
11111       100         1           2:54:43
11111       100         1           2:56:03
11111       100         1           2:56:35
11111       100         1           2:58:51

And this Output table (q):

EmployeeID  EventId     EntityId    ExitDate
11111       100         3           2:52:46
11111       100         3           2:53:29
11111       100         3           2:55:15
11111       100         3           2:57:33
11111       100         3           2:58:15
11111       100         3           6:26:08

With that code:

SELECT DISTINCT TOP (100) PERCENT u.EmployeeId, CAST(p.DeviceTS AS Date) AS Day, p.DeviceTS AS EnterDate, MIN(q.DeviceTS) AS ExitDate
FROM            (SELECT        a.DeviceTS, a.UserId
                      FROM            dbo.DeviceLog AS a INNER JOIN
                                                dbo.Users AS u ON a.UserId = u.Id
                      WHERE        (a.EventId = 100) AND (a.EntityId IN (1, 2, 5, 6, 9, 10, 20, 25, 27))) AS p LEFT OUTER JOIN
                         (SELECT        a.DeviceTS, a.UserId
                           FROM            dbo.DeviceLog AS a INNER JOIN
                                                     dbo.Users AS u ON a.UserId = u.Id
                           WHERE        (a.EventId = 100) AND (a.EntityId IN (3, 4, 7, 8, 11, 12, 21, 26, 28))) AS q ON p.UserId = q.UserId AND CAST(p.DeviceTS AS Date) = CAST(q.DeviceTS AS Date) AND 
                     q.DeviceTS > p.DeviceTS INNER JOIN
                     dbo.Users AS u ON p.UserId = u.Id AND u.EmployeeId IS NOT NULL
GROUP BY u.EmployeeId, p.DeviceTS
ORDER BY u.EmployeeId, EnterDate, ExitDate

Returns this result:

    EmployeeId    EnterDate    ExitDate
1   11111         2:54:11      2:55:15
2   11111         2:54:43      2:55:15
3   11111         2:56:03      2:57:33
4   11111         2:56:35      2:57:33
5   11111         2:58:51      6:26:08

I wanted to do so so that when there were two entries or two exits in a row it would represent the value as null as the following table:

    EmployeeId    EnterDate    ExitDate
1   11111         NULL         2:52:46
2   11111         NULL         2:53:29
3   11111         2:54:11      NULL
4   11111         2:54:43      2:55:15
5   11111         2:56:03      NULL
6   11111         2:56:35      2:57:33
7   11111         NULL         2:58:15
8   11111         2:58:51      6:26:08

Could you do that? Note: I cannot use IF, CASE, etc.