make subject_data more consistent across aggregated and non-aggregated mode

[surchs]

In #214 the subject_data argument return type was changed for the aggregated API mode from list to str like so:

Expected subject_data format in:

• aggregated mode: subject_data = "protected"
• non-aggregated mode:

{
        "sub_id": "sub-123",
        "session_id": "ses-123",
        "session_file_path": "/ds123/sub-123",
        ...
},

There are several good notes on this in the PR (e.g. here).

My concern is that changing the return type from list to string will likely surprise someone somewhere in the future. On the other hand, we also don't want to make it look as if the query just didn't return any results. So I want to open this issue in order to think about this again and maybe find a different implementation that achieves both goals

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