Allow subdir = None

[MarleneKress79789]

Currently, we are not sure how the TE behaves if subdir = None when downloading/uploading/loading a model.

we need tests to check for this, and fix any issues we find.

( to be discussed: we can also explicitly disallow empty subdir.)

tests should be added here:

https://github.com/exasol/transformers-extension/blob/main/test/unit/utils/test_load_local_model.py
https://github.com/exasol/transformers-extension/blob/main/exasol_transformers_extension/utils/bucketfs_model_specification.py
https://github.com/exasol/transformers-extension/blob/main/exasol_transformers_extension/utils/bucketfs_model_specification.py

fixes will likely be needed here:

transformers-extension/exasol_transformers_extension/utils/bucketfs_model_specification.py

Line 32 in cae5e24

def get_bucketfs_model_save_path(self) -> Path:

once this is done, also make sure the new list models udf can deal with empty subdir