RunModel.Supervision: handle NA values on injected flows

As noticed by Myriam Soutif-Bellenger, in case of NAs in flows provided on direct flow nodes, these ones are converted to zeros, leading to unexpected results on simulated flow downstream.

The change to zero flow in case of NAs is operated here: https://gitlab.irstea.fr/in-wop/airGRiwrm/-/blob/dev/R/utils.R#L168