register-nanopore.py throws attribute error if no unclassified folder was provided

[Steffengreiner]

Describe the bug
Trying to upload a nanopore datastructure which contains no unclassified folders leads to following attribute error:

Traceback (most recent call last):
File "../core-plugins/QBIC/1/dss/drop-boxes/register-nanopore-dropbox/register-nanopore.py", line 199, in prepareUnclassifiedData
relativePath = unclassifiedDataObject.getRelativePath()
AttributeError: 'NoneType' object has no attribute 'getRelativePath'

Sadly this leads to an abort of the data upload.

To reproduce
Upload datastructure similiar to the one outlined in the "valid-json" example specified in data-model-lib.

Expected behavious
According to the schema specified in data-model-lib an "unclassified" folder is currently not required. Therefore the upload should run through even if no unclassified folders were provided