[BUG] ARX refuses to load a project containing a possibly empty string value as QID without hierarchy

[jenno-verdonck]

Describe the bug
In the GUI I have a dataset where a certain Quasi-identifier can also have an empty string as value. I didn't set a hierarchy yet as I didn't want to generalize this value while still taking it into account for k-anonymity. I can perfectly anonymize the data with these settings. I saved the project file and later reloaded it. Suddenly I can't load the file and get the following error: hierarchy does not contain a transformation rule for value ''.
After running the source in debug I could already identify 1 difference. It seems that the default empty hierarchy (only containing the unique values without extra levels) that is given to the anonymizer does contain the empty string as a possibility while the hierarchy that is loaded from the project file does not contain this empty string. The code then crashes on the // Register at the dictionary and encode line in the DataManager constructor.

To Reproduce
Steps to reproduce the behavior:

1. Create an anonymization setup where the input data has a Quasi-identifier with empty values.
2. Do not create a hierarchy for this Quasi-identifier.
3. Anonymize the data.
4. Save the project.
5. Try to load the project from the file
6. See the given error

Expected behavior
It should be possible to load/save this type of project.

ARX GUI (please complete the following information):

• OS: Windows
• Version 3.9.1