no progress bar with backend = NULL

[MaximilianPi]

Hi @mihaiconstantin

thanks for the great package. Is it intended that there's no progress bar for backend = NULL?:

parabar::configure_bar(type = "modern", format = "[:bar] :percent :eta", width = getOption("width")/2)
results <- parabar::par_sapply(NULL, 1:100, function(x) {
  # Sleep a bit.
  Sys.sleep(0.01)
  
  # Compute and return.
  x + 1
})