with_distrinct_attribute should accept `None` as a value.

[bidoubiwa]

Description Currently with_distinct_attribute does not accept None as a value. Meaning that once you have set one distinct attribute you need to either user reset_distinct_attribute or create an empty string as value.

https://github.com/meilisearch/meilisearch-rust/blob/6c3a1d2c4acd482c4d57837bb4ee1f74c1d2c0b3/src/settings.rs#L188-L193

Expected behavior None should be a valid type

Current behavior It is not