Skip datalake prefix validation when  datalake format is not null

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.


### Description

If datalake format is not set, skip prefix validation so that users can disable or enable datalake format without re-supplying all datalake-prefixed properties.

### Willingness to contribute

- [x] I'm willing to submit a PR!