Add sanity check for null WebProxy in NuGet.targets
Add sanity check for null WebProxy in NuGet.targets

CSharpFan/Topshelf,CedarLogic/Topshelf,JDTLH9/Topshelf,splatteredbits/Topshelf,phatboyg/Topshelf,naveensrinivasan/Topshelf,CedarLogic/Topshelf,naveensrinivasan/Topshelf,sidecut/Topshelf,phatboyg/Topshelf,tleviathan/Topshelf,Topshelf/Topshelf,pruiz/Topshelf,JDTLH9/Topshelf,sidecut/Topshelf,splatteredbits/Topshelf,tleviathan/Topshelf,pruiz/Topshelf