Unknown parameter: customRanking

I was trying to query using customRanking paramater

```
final SearchForHits queryHits = SearchForHits(
indexName: indexName,
query: searchText,
hitsPerPage: hitsPerPage,
page: page,
filters: filters,
attributesToRetrieve: attributesToRetrieve,
customRanking: ranking);
```

However i get a thrown error
**AlgoliaApiException{statusCode: 400, error: {"message":"Unknown parameter: customRanking","status":400}}**

The ranking is an empty array [].

When i remove the customRanking parameter, it works totally fine.

Start with the SearchForHits entry point shown in the issue and trace how customRanking is added to the request, including the empty-array case. Reproduce the 400 response, then verify that a query using customRanking no longer sends an unsupported parameter and that queries without it still work.

Written by the indexing model from the issue text.