NamedSpecifier wasn't defined to take null for name as intended

An import specifier can either have a name (alias) or not:

```js
// both valid
import {foo as bar} from "foo";
import {foo} from "foo";
```

The definition for `NamedSpecifier` was set to default to null for the name, but wasn't defined to actually accept null as the value. This PR fixes the definition :)