GPII-668: Acceptance test added ensuring that the system doesn't crash on login with empty NP set

Should be reviewed with: https://github.com/GPII/universal/pull/237/files

To do a "before/after" run of the acceptance test by using master and GPII-668 branch, respectively, of kaspermarkus/universal, make sure to add the empty ({}) "empty.json" preferences set exist in the testData/preferences/acceptanceTests folder when running with master, so the fail wont be due to a missing file.