Fix flamby and GUI bug leading to "Error while listing files: undefined"

Fixes #406 (closed) .

The bug was caused by the fact that Flamby datasets are saved in the database with path=None. The reason for this is that the flamby library automatically knows the location of the dataset, hence it makes no sense to also write it in our database (and could lead to errors/misuses by the researcher).

However, I guess we made the implicit assumption that paths would always be defined as something, hence the bug. Obviously, another approach would have been to stop saving flamby datasets with path=None, and setting anything (even empty string). But I think that we should instead make the code robust to the possibility that paths are None, so I prefer my implementation (but I'm open to discussion of course!).

Guidelines for MR review

General:

• give a glance to DoD
• check coding rules and coding style
• check docstrings (eg run tests/docstrings/check_docstrings)

Specific to some cases:

• update all conda envs consistently (development and vpn, Linux and MacOS)
• if modified researcher (eg new attributes in classes) check if breakpoint needs update (breakpoint/load_breakpoint in Experiment(), save_state/load_state in aggregators, strategies, secagg, etc.)