Check for null before overriding task settings

[davidkyle]

Non issue as this fixes a bug in unreleased code

[elasticsearchmachine]

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

[jan-elastic]

I don't really understand this behavior.

If I call model.overrideWith(Map.of()), I expect the settings to be cleared, not to be preserved. Why this unintuitiuve behavior?

[davidkyle]

This code is hit when the inference API is called, the inference endpoint allows the user to override the default model task settings either for experimentation or to change the behaviour in certain situations

POST _inference/text_embedding/my_model 
{
  input: ["....],
  task_settings: {
    "temperature": 5
  }
}

For an setting to be overridden it must be explicitly set.

"task_settings" : {
  "temperature": 5
}

Calling the API with empty task_settings does not explicitly overwrite any options, in this case the user probably didn't mean wipe all the task settings, what does it mean to wipe the defaults anyway as what would we replace them with.

"task_settings" : {
}

[jan-elastic]

I think this kind of randomization is considered bad practice, see: https://github.com/elastic/elasticsearch/blob/main/TESTING.asciidoc#bad-practices

[davidkyle]

I wanted to ensure that a specific code path was taken and was less concerned about which clause in the conditional test was triggered. I created separate test cases for each to be more explicit and the naming of the tests also aids understandability.

[jan-elastic]

LGTM