running with --keep_exact_zeros crashes

Created by: mihofer

Running omc3 with the option --keep_exact_zeros crashes with a ValueError: Index data must be 1-dimensional issue stems from https://github.com/pylhc/omc3/blob/f530c921deaab5f35a071620ce83bf5e76fef5a7/omc3/harpy/clean.py#L124 as it return a pd.DataFrame and not the required empty pd.Index exact zeros check is also skipped in the accuracy tests