Allow null to be cached in memory for unavailable URLs
Allow null to be cached in memory for unavailable URLs

abdul-khalid/LazyList,peterdocter/LazyList,pretizy/LazyList,vaibhav712/imagelist,rsrahulee/LazyList,thest1/LazyList,yukihane/LazyList,xiaojiu01/LazyList,chathudan/LazyList,jiachenning/LazyList