title: User selects "All", nothing passed to filter
body: <p>I'm creating a shiny app which includes a drop down list. The user has a couple of options. These then are used in a filter as shown below.</p>
  
  <pre><code>filteredData &lt;- reactive({
  
  data %&gt;% 
    filter(Month &gt;= input$months[1],
           Month &lt;= input$months[2],
           Location.ID == input$loc,
           )
  })
  </code></pre>
  
  <p>I want to add "All" in the drop down associated with input$loc and then want nothing to be passed to the filter.</p>
  
  <p>I'm not really sure what I need to pass to the filter so it doesn't do anything. I tried NULL but it doesn't seem to work.</p>