Fix for null string issue reported by tikuf
Fix for null string issue reported by tikuf

Did some extra null checking to avoid marshalling a null string across
to native code.

heksesang/Emby.Plugins,SvenVandenbrande/Emby.Plugins,heksesang/Emby.Plugins,hamstercat/Emby.Plugins,lalmanzar/MediaBrowser.Plugins,MediaBrowser/Emby.Plugins,Pengwyns/Emby.Plugins,Pengwyns/Emby.Plugins,lalmanzar/MediaBrowser.Plugins,SvenVandenbrande/Emby.Plugins,MediaBrowser/Emby.Plugins,heksesang/Emby.Plugins,hamstercat/Emby.Plugins,SvenVandenbrande/Emby.Plugins