SQL SUM return 0 where no rows found

Select DH.DH_Name,Z.loc_name,
sum(NVL(si.si_income1,0)) as Value
From (select ROW_NUMBER () OVER (partition by LGT_DISPLAY_ORD ORDER BY LGT_DISPLAY_ORD DESC ) as position, 
        l.loc_id, 
        l.loc_loc_code, 
        l.loc_name, 
        t.LGT_SHORT_CODE, 
        t.LGT_DESCRIPTION as area, 
        t.LGT_REGION, 
        l.LOC_DATE_OPENED, 
        l.LOC_DATE_CLOSED, 
        store_status( l.LOC_DATE_OPENED, l.LOC_DATE_CLOSED) as status
        from LOCATION_GROUP_TYPES t, LOCATION_GROUPS g, locations l
        where t.LGT_TYPE = 'R' 
        and t.LGT_ID = g.LG_LGT_ID 
        and LG_LOC_ID = l.LOC_ID
        and (LOC_DATE_CLOSED is null or to_char(LOC_DATE_CLOSED,'YYYY')  = 2015)
        and l.loc_loc_ty_id = 4   
        order by t.LGT_DISPLAY_ORD, l.loc_loc_code) Z
inner join SITE_INCOME si
on si.si_loc_id = Z.loc_id
inner join Department_Headings DH
on DH.DH_Position = 1
Where si.si_year =  2015
and si.si_week Between 24 and 24
AND Z.position = 6 and Z.Area = 'NORTH EAST'
group by DH.DH_Name,Z.loc_name

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