...
Note that if the record in your collection doesn’t have values for a field, then those values will not be returned in the response. For example, if one record doesn’t have published_time value, it will still be returned in response with title and url.
If you want to only return fields that have all these three values, then you will have to use filters (e.g. "filter":"title!='',url!='',published_time>'2000-01-01'")
...
Documentation
Related articles
...