c# linq left join null值

from djb in djbEntity.Select(d => new { d.ZSLX, d.Id })
                              join fw in fwEntity.Select(d => new { d.SLBH })
                              on djb.Id equals fw.SLBH into fwtemp
                              from tt in fwtemp.DefaultIfEmpty()
                              where tt.SLBH == "" || tt.SLBH == null

where條件必須是""與null兩個條件，如果單獨為null則進行全盤掃描速度奇慢，如果單獨為""則查詢不出結果。