results in one line

Hi there. i would like to ask for some suggestions. i have an output below. Now i have the startdate and End date. i Would like to ask if i can possibly joing them in one line or avoid showing those null values since my querry is this:\

select person.supervisor [Supervisor ID],
sup.displayname [Supervisor Name],
person.personid [Technician ID],
person.displayname [Technician Name],
wostatus.wonum [Work Order No],
--wostatus.status [Status],
StartDate =
 Case
  When wostatus.status = 'Created' then wostatus.changedate 
 end, 
EndtDate =
 Case
     When wostatus.status = 'Comp' then wostatus.changedate
 end

from wostatus
inner join workorder on wostatus.wonum = workorder.wonum
inner join person on person.personid = workorder.reportedby
left outer join person sup on sup.personid = person.supervisor
where istask = 0  and
worktype in ('CM','SPM')   and  
wostatus.status in ('Created', 'COMP') and
workorder.status in ('COMP','CLOSE')  
and workorder.wonum = '10007'