getCurrentOutputInfo(): return NULL if not in an output

[wch]

This is related to rstudio/DT#730.

Previously, this app would throw an error when someone visited it, so the app session wouldn't even start for that user.

library(shiny)
shinyApp(
  ui = fluidPage(
    verbatimTextOutput("txt1"),
    verbatimTextOutput("txt2")
  ),
  server = function(input, output) {
    outside_info <- getCurrentOutputInfo()
    output$txt1 <- renderPrint(outside_info)
    
    output$txt2 <- renderPrint( getCurrentOutputInfo() )
  }
)

After this change, it displays this:

[schloerke]

The JS travis failure might be due to outdated module deps. Maybe try to run cd tools && yarn && yarn build && cd ../

Returning null seems fair as no info is available. Having an empty list seems more misleading.