title: plotly::renderPlotly() does not allow for NULL values. How can I workaround?
body: <p>Title says it basically. For a complex Shiny App I have to be able to send NULL values to <code>renderPlotly()</code>, because I want to display a plot only when certain conditions are met. The normal <code>shiny::renderPlot()</code> is able to do this.
  A small example which gives an error I do not want:</p>
  
  <pre><code>library(shiny)
  library(plotly)
  
  ui &lt;- fluidPage(
    plotlyOutput("plotly"),
    plotOutput("plot")
  )
  
  server &lt;- function(input, output) {
  
    # I need to be able to put NULL in here or anything so that
    # there is no output and also no error message.
    output$plotly &lt;- renderPlotly({
      NULL
    })
  
    # This works and sends no error message
    output$plot &lt;- renderPlot({
      NULL
    })
  
  }
  
  shinyApp(ui, server)
  </code></pre>
  
  <p>Note that the Web App displays only one error message, the one from <code>renderPlotly()</code>.
  I am looking for any workaround to this. How can I switch between those two plots which should be displayed at the same spot in the app, and always ignore one of them, depending on other input?</p>