Colour legend is broken in scatterplot where only one value exists and a colorbar title is provided

Example:

library(plotly)
df <- data.frame(x = 1:5, y = 1:5, col = c(1,1,1,1,1))
plot_ly(df, x = ~x, y = ~y, color = ~col) %>%
    add_markers(marker = list(colorbar = list(title = "test")))

Result:

If any of the col values changes and it is no longer only 1's, then it works as expected. Also, if the title parameter is omitted, it also works as expected.

Here's the result when I omit the title:

This is using plotly 4.8.0 - latest on CRAN

Run the provided plotly R example with a constant col value and a colorbar title, then compare it with the versions where the title is omitted or col varies. Trace the behavior of the color legend and colorbar title, and consider the issue done when the constant-value scatterplot renders the title correctly without breaking the legend.