search_data_catalog crashes if the result is empty

[RondeauG]

Setup Information

• xscen version: 0.2.3

Description

If search_data_catalog fails to find valid entries, the code crashes instead of returning a warning and/or an empty dictionary.

Steps To Reproduce

 cat = xs.extraction.search_data_catalogs(DC, variables_and_freqs={"tas": "D"}, other_search_criteria={"source": "make_it_crash"})

Will result in:

Traceback (most recent call last):
  File ".../pycharm-community-2017.3.4/helpers/pydev/_pydevd_bundle/pydevd_exec2.py", line 3, in Exec
    exec(exp, global_vars, local_vars)
  File "<string>", line 1, in <module>
  File ".../xscen/config.py", line 163, in _wrapper
    return func(*args, **kwargs)
  File ".../xscen/extraction.py", line 149, in search_data_catalogs
    for (sim_id,), scat in catalog.iter_unique("id"):
  File ".../xscen/catalog.py", line 198, in iter_unique
    for values in itertools.product(*map(self.unique, columns)):
  File ".../xscen/catalog.py", line 186, in unique
    raise ValueError("Catalog is empty.")
ValueError: Catalog is empty.

Additional context

No response

Contribution

• I would be willing/able to open a Pull Request to address this bug.