Make it possible to reset some specific settings via the updateSettings route

Nobody has claimed this yet.

**Description**
For example, it is possible to reset the synonyms to it' default value by sending

```json
{
"synonyms": null
}
```

As the json body of the `updateSettings` method.

Unfortunately, looking at this code:

https://github.com/meilisearch/meilisearch-java/blob/04244faad4b653fe4cdc5eff871463ce6e8b83bc/src/main/java/com/meilisearch/sdk/Settings.java#L33-L60

It appears that if the user sets `synonyms` to null it will not be added to the final body send to `updateSettings`, and thus, it will not reset the synonyms.

As [per the documentation](https://github.com/meilisearch/meilisearch-java/blob/04244faad4b653fe4cdc5eff871463ce6e8b83bc/src/main/java/com/meilisearch/sdk/Settings.java#L33-L60)

> Passing null to an index setting will reset it to its default value.

It may not be possible to do it as we cannot know the difference between a field not present or a field set specifically to null. Nonetheless if it is possible, it should be investigated.

Start in src/main/java/com/meilisearch/sdk/Settings.java around lines 33-60 and trace how updateSettings builds its JSON request. Determine whether explicitly supplied null settings can be distinguished from omitted settings, then verify that null resets a setting while omitted fields remain unchanged.

Written by the indexing model from the issue text.