Previously if the IDL was something like:
```
constructor(optional DOMString data = "");
```
We were generating code that would be passing through to the constructor
an Optional<String> - even though for this situation it is not possible
for it to be null.
Instead, if we know if there is a default value that is non-null and the
type is not nullish, just generate the cpp code as a String.
914fb90bbe