Unmatched null pattern

https://github.com/HaxeFoundation/format/blob/master/format/swf/Writer.hx#L547

```
bits.writeBit(switch(data.fill) {
case LS2FColor(_): false;
case LS2FStyle(_): true;
});
```

https://github.com/HaxeFoundation/format/blob/master/format/swf/Writer.hx#L570

```
switch(data.fill) {
case LS2FColor(color): writeRGBA(color);
case LS2FStyle(style): writeFillStyle(ver, style);
}
```

The error is correct because LS2Data defines fill to be `Null` and Haxe now considers explicit null types for exhaustiveness.

The question is what should semantically happen if the field actually is null. Or should the fill field not be nullable after all?