Don't fail in the shiny progress handler before the bar is shown (#762)

[taekop]

cli_progress_output() errors with "attempt to apply non-function" under the Shiny progress handler when the bar hasn't been shown yet:

shiny::testServer(function(input, output, session) {
  id <- cli::cli_progress_bar(total = 10)
  cli::cli_progress_output("custom message", id = id)
}, {})

(with options(cli.progress_handlers_only = "shiny"); in a real app the handler is picked automatically.)

Bars are only added after cli.progress_show_after (2 seconds by default), and the handler's output() method called bar$shiny_progress$set(), but shiny_progress is only created in add(). So any output sent before that point hits NULL$set().

output() still records the text in bar$shiny_output, so it shows up once the bar is added; it just doesn't call set() until there is a Shiny progress object.

The new test in test-progress-handlers.R fails on main and passes here. I also tried output before and after the bar is shown, several outputs, cli_progress_message(), cli_progress_step() and finishing before the bar is ever shown; only the calls that used to error behave differently. The only failures in the full suite locally are the keypress() tests, which fail the same way on main.

I added shiny to Suggests for the test.

Closes #762