Restoring past trials from storage fails when experiment has no tunables

[motus]

When experiment has no tunables at all, restoring data from the storage crashes with the following exception:

File "/root/.local/lib/python3.12/site-packages/streamlit/runtime/fragment.py", line 246, in wrapped_fragment
    result = non_optional_func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/root/.local/lib/python3.12/site-packages/dashboard/main.py", line 29, in details_page
    exp_and_df = widgets.experiment_picker(storage, "details_page_exp_picker")
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/root/.local/lib/python3.12/site-packages/dashboard/widgets.py", line 248, in experiment_picker
    df = experiment.results_df
         ^^^^^^^^^^^^^^^^^^^^^
File "/root/.local/lib/python3.12/site-packages/mlos_bench/storage/sql/experiment_data.py", line 212, in results_df
    return common.get_results_df(self._engine, self._schema, self._experiment_id)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/root/.local/lib/python3.12/site-packages/mlos_bench/storage/sql/common.py", line 183, in get_results_df
    ExperimentData.CONFIG_COLUMN_PREFIX + row.param_id,
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str

We need to handle this situation gracefully and add unit tests to check for this condition.