jsii methods/constructors can have optional arguments. The .NET generator should respect those and pass null in calls that don't supply them.

-- from @mpiroc:

jsii-dotnet-generator should supply default values for arguments with optional types

jsii-dotnet-generator ensures that null is a valid value for types marked optional in JSII. For value types, this means making them nullable, i.e. int?. For reference types, null is already a valid value.

When a type like this is used in a parameter list, jsii-dotnet-generator should create a default value of null for the parameter. Currently, the user has to explicitly pass null.