Regression in `MAP` literals for `GenericDialect`

## Summary

`MAP { ... }` literals are failing with `GenericDialect`. Example:
```
SELECT MAP {'a': 1, 'b': NULL}
```

Fails with:
```
ParserError("Expected: end of statement, found: {")
```

This is causing failures in DataFusion tests when trying to upgrade: https://github.com/apache/datafusion/pull/20177

It's a regression from the last release (v0.60.0).