Fix FacetSearchResults when passing an empty facet_query string.

When we pass an empty "facetQuery" param to Meilisearch, the response turns the empty
string into a null value. This causes "Input should be a valid string" errors, making it
impossible to run this query (which is quite a useful one, since it returns all the
distinct values for an index.)

Author: diegov

meilisearch_python_sdk/models/search.py

@@ -18,7 +18,7 @@ class FacetHits(CamelBase):
 
 class FacetSearchResults(CamelBase):
     facet_hits: list[FacetHits]
-    facet_query: str
+    facet_query: str | None
     processing_time_ms: int