Added the delete test for NullAdapter
Added the delete test for NullAdapter

hannesvdvreken/flysystem,dshafik/flysystem,oswaldderiemaecker/flysystem,mapyo/flysystem,mhlavac/flysystem,farmisen/flysystem,sachintaware/flysystem,sohelrana820/flysystem,barryvdh/flysystem,twistor/flysystem,mhlavac/flysystem,Amrit01/flysystem,nao-pon/flysystem,twistor/flysystem,CarsonF/flysystem,dstockto/flysystem,concrete5/flysystem,rossriley/flysystem