Avoid None for empty results

[diethard-at-alation]

Sometimes GET requests won't return data. Currently in quite a few cases we are just returning None, which is not very convenient (e.g. get_data_quality_values).

Proposed solution: Even if no results could be fetched, return the same data type as if actual results are available. Example: for get_data_quality_values we would return an empty list.
Make this consistent across the board.