flatten_timepoint_specific_output_overrides should not do anything if there are no timepoint-specific output parameters

[dweindl]

flatten_timepoint_specific_output_overrides's only purpose is getting rid of timepoint-specific output parameters. If there aren't any, it shouldn't modify the petab problem. Currently, the problem is modified nonetheless.

Depending on the exact way of fixing this, unflatten_simulation_df will probably also have to be adapted.