Fix ConstantUser() ctor. Use null and not 0 as value. 0 is used when saving null.
Fix ConstantUser() ctor. Use null and not 0 as value. 0 is used when saving null.

jorik041/dnlib,picrap/dnlib,ilkerhalil/dnlib,yck1509/dnlib,ZixiangBoy/dnlib,Arthur2e5/dnlib,kiootic/dnlib,0xd4d/dnlib,modulexcite/dnlib