title: Shiny selectizeInput: on click remove the initial selected value
body: <p>i would like to remove the initial value (<code>selected=</code>) from <code>selectizeInput</code> when the user click on the widget.</p>
  
  <p>Here is a sample code:</p>
  
  <pre><code>library(shiny)
  library(dplyr)
  
  ui= fluidPage(
    sidebarLayout(
      sidebarPanel(
        selectizeInput(inputId= "cyl", label= "cyl", 
                       choices= NULL, 
                       selected= sort(unique(mtcars$cyl))[1],
                       multiple=T)
      ),
      mainPanel(
        tableOutput("tab")
      )
    )
  )
  
  server= function(input, output,session) {
  
    updateSelectizeInput(session = session,inputId ="cyl",choices=sort(unique(mtcars$cyl)),selected=sort(unique(mtcars$cyl))[1], server = TRUE)
  
    df_filtered= reactive({
      mtcars %&gt;%
      {if (is.null(input$cyl)) . else filter(., cyl %in% input$cyl)}
    })
    output$tab= renderTable(df_filtered())
  }
  
  shinyApp(ui, server)
  </code></pre>
  
  <p>Just a bit of explanation on base of sample code:</p>
  
  <p>The initial selected value in <code>selectizeInput "cyl"</code> is <code>4</code>. When the user press on this widget, i would like that the value <code>4</code> <strong>is removed</strong> and the <code>selected</code> option is cleared. Any ideas? </p>
  
  <p>*I have used the function <code>updateSelectizeInput</code> in <code>server</code> because in my <code>shiny app</code> choice selection is very big leading to too long loading time </p>