[converter] implement nullable types
[converter] implement nullable types

Rpinski/RefactoringEssentials,olathunberg/RefactoringEssentials,icsharpcode/RefactoringEssentials,SSkovboiSS/RefactoringEssentials,olathunberg/RefactoringEssentials,SSkovboiSS/RefactoringEssentials,icsharpcode/RefactoringEssentials,Rpinski/RefactoringEssentials