Fix static-analyzer error: +[DDLog registeredClasses] returning nil
Fix static-analyzer error: +[DDLog registeredClasses] returning nil

+[DDLog registeredClasses] should return empty array rather than nil.
Is declared between NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END

sushichop/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,jum/CocoaLumberjack,sushichop/CocoaLumberjack,sushichop/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,DD-P/CocoaLumberjack,jum/CocoaLumberjack,sushichop/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,CocoaLumberjack/CocoaLumberjack,jum/CocoaLumberjack,jum/CocoaLumberjack,DD-P/CocoaLumberjack,DD-P/CocoaLumberjack