[#664] Check that source question exists when publishing survey

* verify existence of the source question from which the source `surveyId` is retrieved
* AFAK `qList` can't be `null`, but just an empty list.
* IMHO it's easier to just check for `null` for the return value of `getByKey()`, e.g.

````java
Question sourceQuestion = questionDao.getByKey(question.getSourceId())
if (sourceQuestion != null) { return String.valueOf(sourceQuestion.getSurveyId()); }
````