title: Navigator showing at 1970 when first point is null
body: <p>I've got data being loaded asynchronously from a client. From time to time, null time and value pairs will be received. If this happens, the navigator shows beginning at 1970. 
  I've tried setting the initial point to the first timestamp in the navigator series and the problem persists, as the null is in the series being displayed.</p>
  
  <p>Is there a solution you can suggest or a workaround?</p>
  
  <p>Here's my code and <a href="http://jsfiddle.net/Tt7dp/" rel="nofollow">a fiddle</a>:</p>
  
  <pre><code>$(function() {
      $('#container').highcharts('StockChart', {
  
          chart: {
          },
  
          navigator: {
              adaptToUpdatedData: false,
              series: {
                  name: 'navigator'
              }
          },
  
          rangeSelector: {
              enabled: false
          },
          series: [{
              name: "Billy"
          }
          ]
      });
  
      var data = [[null, null],
                      [Date.UTC(1990,  9, 27), 10 ],
                      [Date.UTC(1990, 10, 10), 20 ],
                      [Date.UTC(1990, 10, 18), 30 ],
                      [Date.UTC(1990, 11,  2), 40 ],
                      [Date.UTC(1990, 11,  9), 50 ],
                      [Date.UTC(1990, 11, 16), 60 ],
                      [Date.UTC(1990, 11, 28), 50],
                      [Date.UTC(1991,  0,  1), 40],
                      [Date.UTC(1991,  0,  8), 30],
                      [Date.UTC(1991,  0, 12), 20],
                      [Date.UTC(1991,  0, 27), 10]]
      var chart = $('#container').highcharts();
      chart.series[0].setData(data);
      chart.xAxis[0].setExtremes(chart.xAxis[0].min, chart.xAxis[0].max, true);
  });
  </code></pre>