Parser recovers badly when function declaration is missing identifier

`function () {}` (a statement, not an expression) errors with an "unexpected token (" because it's missing the function name. but it recovers by making an `Identifier` with name `(`, which is also illegal.

```
{
"type": "Identifier",
"name": "(",
"typeAnnotation": null,
"optional": false
}
```

maybe instead it should use an Identifier with name `""`. or, we allow `FunctionDeclaration` to have a null id for `export default function() {}`, so maybe it's slightly less invalid to use null instead of an invalid identifier.