add_ribbons can't handle NA values

The plotly function add_ribbons() doesn't function properly when cases in the middle or at the end have NA values. These lead to a weird behaviour where parts of the ribbons are missing or drawn in wrong places (see Plots 1 & 3). When the connectgaps-Attribute is set to TRUE for the ribbon the bug doesn't appear (see Plot 2).
NA values in the beginning don't seem to be a problem as well (see Plot 3, Red Trace 2).

The same behaviour has also been reported with ggplot and plotly in issue #1060. But because it may either be two separate problems or just a problem in plotly itself also affecting the ggplot visualization I decided to open this separate issue.

Inizia con la funzione R add_ribbons() e riproduci il comportamento utilizzando i Plots 1–3 collegati, concentrandoti sui valori NA al centro e alla fine. Confronta il risultato con connectgaps impostato su TRUE e con la issue #1060 correlata di ggplot e plotly; il lavoro è terminato quando le ribbons vengono renderizzate nei punti corretti senza richiedere connectgaps.