fix(request_settings): don't send empty string for request_condition

[Integralist]

The API will consider the service invalid if it has multiple instances of request_setting with the same request_condition attribute value.

This PR doesn't fix that issue. This PR change is here because it's the correct thing to do.

It doesn't help though (even though we're not sending request_condition to the API if it's an empty string) because the API itself still assigns an empty string as a default for each request_setting object, and so later on in the Terraform flow when the service is 'validated' (i.e. GET /service/<SERVICE_ID>/version/<VERSION>/validate) it will return an error because it sees there are multiple request_setting objects each with an empty string for the request_condition attribute.