title: Shiny numeric input with multiple option and NULL as initial
body: <p>In Shiny <code>selectizeInput</code>, if nothing is selected, it returns <code>NULL</code>.</p>
  
  <p>I wonder if there is a similar update version for numericInput which takes multiple values, e.g. <code>c(2,4)</code> or <code>5:9</code>, and returns <code>NULL</code> for missing input?</p>