don't to perform download if url is "" but not nil
don't to perform download if url is "" but not nil

sometimes user may pass @"" as url to sdwebimage to download, now it will also run NSURLConnection and try to download, actually there is no need to do this. I think maybe use url.absoluteString.length == 0 to avoid that url is not nil but a empty string will be better
ikesyo/SDWebImage,twobitlabs/SDWebImage,zllbird/SDWebImage,masterfego/SDWebImage,rs/SDWebImage,ruaho/SDWebImage,Insfgg99x/SDWebImage,dreampiggy/SDWebImage,awkward/SDWebImage,yunzhongyoulong/SDWebImage,dpettigrew/SDWebImage,HannaLu/SDWebImage,iheartradio/SDWebImage,altrooz/SDWebImage,nickhingston/SDWebImage,bpoplauschi/SDWebImage,dreampiggy/SDWebImage,amyhametov/SDWebImage,mattjgalloway/SDWebImage,klundberg/SDWebImage,odyth/SDWebImage,awkward/SDWebImage,zhang-yawei/SDWebImage,iMinichrispy/SDWebImage,heroims/SDWebImage,tokorom/SDWebImage,cochat/SDWebImage,Kekanto/SDWebImage,geoffmacd/SDWebImage,ttgb/SDWebImage,Deyine/SDWebImage,tualatrix/SDWebImage,iosnatarajan/SDWebImage,Frntbac/SDWebImage,smyrgl/SDWebImage,smule/SDWebImage,irealme/SDWebImage,buleSky111/SDWebImage,awkward/SDWebImage,dreampiggy/SDWebImage,initlifeinc/SDWebImage