title: Creating a null value in a DATE validated column
body: <p>I have a uiApp that is creating table views from spreadsheet data, but I'm having difficulty with empty cells in a date column.</p>
  
  <p>Basically, some of the spreadsheet cells are optional, and I haven't figured out how to put no (or a null) value into the table. Right now it works if I set the column value to:</p>
  
  <pre><code>projectDataRaw[i][projectDataKeys[j]]= new Date(); // inserting dummy data
  </code></pre>
  
  <p>But I want to leave those cells blank in the table view too, however, when I use:</p>
  
  <pre><code>projectDataRaw[i][projectDataKeys[j]]= null; // 
  </code></pre>
  
  <p>Or some variation of that, it errors because the column type is set to DATE.
  I could convert to strings, but I kind of wanted to be able to compare date values to each other when they are available.</p>
  
  <p>-- Example of issue with table view and null data --</p>
  
  <pre><code>// load data from sheet
  var _PMsheet = "sheet key";
  var sSheet = SpreadsheetApp.openById(_PMsheet);
  var projectData = sSheet.getRangeByName("ProjectDateCol").getValues();
  
  var projectDataTable = Charts.newDataTable();
  projectDataTable.addColumn(Charts.ColumnType.DATE, "Date Column");
  
  for (var i=0; i &lt; projectData.length; i++) {
    if (projectData[i] == undefined || projectData[i] == "undefined") {
      projectDataTable.addRow([null]);// Object type does not match column type.
      // null fails, "undefined" fails, "" fails, [] fails 
    }
    else {
      projectDataTable.addRow([projectData[i]]); // this cell has a valid date obj
    }
  
  }
  </code></pre>