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.

meilisearch-rust/src/settings.rs

Lines 188 to 193 in 6c3a1d2

	pub fn with_distinct_attribute(self, distinct_attribute: impl AsRef<str>) -> Settings {
	Settings {
	distinct_attribute: Some(distinct_attribute.as_ref().to_string()),
	..self
	}
	}

Expected behavior
None should be a valid type

Current behavior
It is not