[ML] Update Inference Settings cannot clear settings

[prwhelan]

Unsetting or explicitly setting a value to null does not clear the setting:
https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/cohere/embeddings/CohereEmbeddingsTaskSettings.java#L116

All of our updatedTaskSettings should check if a user is explicitly setting a field to null and, if so, clearing the field.

If a user creates an endpoint with input_type set:

    "task_settings": {
        "input_type": "search"
    }

An update like this should clear the input_type setting:

    "task_settings": {
        "input_type": null
    }

Whereas an update like this should not:

    "task_settings": {
    }

Workaround is to delete and recreate the endpoint with the desired task_settings

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)