When JSONWS remote service returns an int value, it is not recognized by liferay.js and the error "The server returned an empty response" is shown.
+ go to API page
+ choose /assetcategory/get-vocabulary-categories-count
+ enter groupId and vocabularyId
+ click on Invoke
The result is: "The server returned an empty response" instead of 0. The server returns 0 correctly, so its on JS side. See attached screenshot.