Export: Check if empty responses from API's during export are handled gracefully

[ValentinFutterer]

During export, several API's are called to get up to date information. The project-DB is one of them, but there are probbaly also others. Since e.g. projects could get deleted, while someone works on a dmp, the question came up, if this case is handled in the project export.

To complete this issue, it should be made sure, that empty responses are handled correctly in all API calls that happen during exporting. If this is not handled correctly, it should be fixed in the code.