ggplotly error when xlab is element_blank()

[doorisajar]

Reprex:

library(ggplot2)
library(plotly)

p <- ggplot(iris, aes(Petal.Length, Sepal.Width, color = Species)) + 
    geom_point() + 
    xlab(element_blank())

ggplotly(p)
#> Error in if (nchar(axisTitleText) > 0) {: argument is of length zero

That seems to be happening here:

https://github.com/ropensci/plotly/blob/ebb017a6fbd1a582787017f98e8e40cb4f1431b3/R/ggplotly.R#L817

xlab("") works, so the issue is definitely that nchar of a theme element that is set to element_blank() is NULL. The same check happens a few lines later:

https://github.com/ropensci/plotly/blob/ebb017a6fbd1a582787017f98e8e40cb4f1431b3/R/ggplotly.R#L839

I get the same error when setting ylab and ggtitle to element_blank, but not for setting the legend title to blank via theme(legend.title = element_blank()).

I might be able to do a pretty minimal PR for this. The tests look reasonably straightforward.

[doorisajar]

I submitted a PR for this: #1732