with_distrinct_attribute should accept `None` as a value.

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

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