LuaMember AllowNil support

[ashtonmeuser]

Allows LuaMembers that hold LuaObjects to accept nil assignment from Lua via [LuaMember("my_prop", AllowNil = true)]. This aligns with the C# concept of said LuaObjects being reference types, and thus, accepting null

[LuaObject]
public partial class AllowNilReferencedObject
{
    [LuaMember("label")]
    public string Label { get; set; } = "";
}

[LuaObject]
public partial class AllowNilMemberContainer
{
    [LuaMember("optionalObject", AllowNil = true)]
    public AllowNilReferencedObject OptionalObject { get; set; } = null!;

    [LuaMember("requiredObject")]
    public AllowNilReferencedObject RequiredObject { get; set; } = null!;
}

state.Environment["target"] = new AllowNilMemberContainer();
state.Environment["reference"] = new AllowNilReferencedObject();

target.optionalObject = reference -- Works
target.optionalObject = nil -- Works
target.requiredObject = reference -- Works
target.requiredObject = nil -- Fails

In C#, state.Environment["target"]. OptionalObject is null.