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.

从 R 的 add_ribbons() 函数开始，使用链接的 Plots 1–3 复现其行为，重点关注中间和末尾的 NA 值。将结果与 connectgaps 设置为 TRUE 时的结果以及相关的 ggplot 和 plotly issue #1060 进行比较；当 ribbons 在正确的位置渲染且不需要 connectgaps 时，即视为完成。