integer division or modulo by zero

Unfortunately the latest bugfix caused the 'mini' configuration (containing only a single simulation) to fail with integer division or modulo by zero in the plotting step.

The error is raised here: https://git.geomar.de/digital-earth/de-climate-change/de-climate-change-analysis/-/blob/master/climateanalysis/plotting/generate_plot.py#L394

Might be caused by a NaN value in the 'series' data array.

@christine-nam maybe you can look into it. Here the corresponding statistics files: statistics_a913c116.zip