[#664] Check existence of source question before retrieving survey id

* changes made based no review by `@iperdomo`
[#664] Reformatting source code
[#664] Merge branch 'issue/664' of https://github.com/akvo/akvo-flow into issue/664

Conflicts:
	GAE/src/org/waterforpeople/mapping/app/web/SurveyAssemblyServlet.java
[#664] Ignore `sourceSurvey` attribute if there is no `sourceId` in a question
Merge pull request #694 from akvo/issue/664

[#664] Fix NPE when no `sourceId` exists