Definition of part6_threshold_combi when more than one set of thresholds fail

[jhmigueles]

When part6_threshold_combi is NULL and multiple threshold combinations can be created using threshold.lig, threshold.mod, and threshold.vig, check_params does not assign the first combination to part6_threshold_combi. Instead, it generates multiple threshold combinations. This behavior causes issues later in part 6 analyses and in identifying relevant files for the visual report.