Fix Issue 10915 - std.typecons.Nullable throws in writeln() if it's null

https://issues.dlang.org/show_bug.cgi?id=10915
Prefer the version that takes a sync, and doesn't allocate:
```D
void toString(scope void delegate(const(char)[]) sink);
```
Ideally, since it's a wrapper, use fmt flags:
```D
void toString(scope void delegate(const(char)[]) sink, FormatSpec fmt);
```
Then use `formatValue` to write onto the sink.