Fix #368: generate proper expression for null-coalescing operators on nullable types

[metoule]

The expression we generate for the null-coalescing operator doesn't work for nullable types, because there's no comparison operator between a nullable type and an object:

const object nullConst = null;
int? MyVar = null;
return MyVar == nullConst ? 0 : MyVar; // <-- doesn't compile

This PR changes the generated expression for nullable types from expr == null ? exprRight : expr to expr.HasValue ? expr.Value : exprRight

Fix #368

[metoule]

Back to draft, so that I can see how to handle the other binary operators.

[metoule]

After playing with sharplab, I discovered that for nullable types, the .NET compiler generates:

int? a;
int b = a ?? 10; // int b = a.GetValueOrDefault(10);

int? a;
bool b = a > 10; // bool b = (a.GetValueOrDefault() > 10) & a.HasValue

I'll update the PR to match this.