Fix handling of parameters with default null

[paramamue]

This in an attempt of improving handling of nullable parameters.
In our solution at work, all projects have <Nullable>disable</Nullable>. In some of the generated interfaces, we receive the warning warning CS8625: Cannot convert null literal to non-nullable reference type., because the interfaces are generated with #nullable enable and have parameters like string input = null.
I've changed the generator so that these parameters activate the nullable mode and add a ? if it's missing.
There were some changes in the tests I didn't expect, e.g. Misc.CustomNameSpace, but I think they're all correct like this.

Note that for 100% correctness, I think we would need to check the semantic model for every method and get the nullable context to decide if a parameter is actually nullable or not.