Empty code editor on the Data Page will crash the app

[stevechoy]

If the placeholder code filter() %>% mutate() is deleted by the user and then the "Apply" button is pressed, the app will crash.

This is due to the logic inside the safely_nmdata_code reactive, where the entire code string (input$codes) is parsed indiscriminately:

nmdata_global     <<- nmdata() # globally assigned as shinyAce editor is not scoped to access this environment
parsed_text <- safely_parse(text = paste('nmdata_global <<- nmdata_global %>% ', input$codes))

Ideally, there should be a check to prevent the app from crashing, but a straightforward solution is not apparent.