title: Tabulator throws error when adding an empty row with datetime formatter
body: <p>I have the following code (just a button, one column and one row)</p>
  <pre><code>    &lt;div class='btn-toolbar'&gt;
          &lt;button id='new' class='btn buttons m-2 mdi mdi-plus' onclick=&quot;NewClick()&quot; type=&quot;button&quot;&gt;New&lt;/button&gt;
      &lt;/div&gt;
      &lt;div id=&quot;example-table&quot;&gt;&lt;/div&gt;
  &lt;script&gt;
      // Tabulator
  
      var tblTestTabelle_40028804 = new Tabulator(&quot;#example-table&quot;, {
          height: &quot;311px&quot;, selectable:true, 
          &quot;data&quot;: [
              {
                  &quot;1A27078D-D7D4-434E-AD90-D825276756FC&quot;: &quot;06.03.2021&quot;,
                  &quot;id&quot;: &quot;3432938F-A7AB-4F12-B756-E645093D4439&quot;
              }
          ],
          &quot;columns&quot;: [
              {
                  &quot;title&quot;: &quot;Date&quot;,
                  &quot;field&quot;: &quot;1A27078D-D7D4-434E-AD90-D825276756FC&quot;,
                  &quot;formatter&quot;: &quot;datetime&quot;, formatterParams:{
                      inputFormat:&quot;dd.MM.yyyy&quot;,
                      outputFormat:&quot;DD&quot;,
                      invalidPlaceholder:&quot;(invalid date)&quot;,
                  }
                  
              }
          ],
          layout: &quot;fitData&quot;
  
      });
      //basic functionality
      function NewClick() { tblTestTabelle_40028804.addRow(); };
  &lt;/script&gt;   
  </code></pre>
  <p>When I click the NewButton I get the following JS exception:
  <code>Uncaught (in promise) Error: fromFormat requires an input string and a format</code> (in luxon.min.js)
  It sounds to me like a bug in tabulator, that the datetime formatter isn't able to handle adding empty rows, but maybe I'm wrong.</p>
  <p>Any help would be appreciated.</p>