[Dart] Support for optional scalar types

It seems that optional scalar types are not supported for Dart

```
❯ ./flatc --dart --gen-object-api -o lib flatbuffer/*

error:
/home/arie/Workspace/eab-solutions/Farmcon/liblink/flatbuffer/animals.fbs:12: 35: error: Optional scalars are not yet supported in at least one of the specified programming languages.
```

I couldn't find any reference to this in the documentation or in existing issues.

Are there any plans to support them in the future?

P.S. I am running on the latest flatc version:
```
❯ ./flatc --version
flatc version 25.9.23
```