function TestFindBlankRow() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sh=ss.getSheetByName('Sheet1');
  //Column A to T because you have formula in column U
  var Rslt=FirstBlankRange(sh,'A','T');  //0 : If Not Found
  Logger.log(Rslt);
  var Rslt=NextBlankRange(sh, Rslt, 'A','T');  //0 : If Not Found
  Logger.log(Rslt);
  var Rslt=LastBlankRange(sh,'A','T');  //0 : If Not Found
  Logger.log(Rslt);
};
function FirstBlankRange(Sheet, FirtColumn, LastColumn)
{
  for (a=1;a<Sheet.getLastRow()+1;a++)
  {
    var rr=Sheet.getRange(FirtColumn + a + ':' + LastColumn + a);
    var Rslt=rr.getValues().toString();
    if ((Rslt.length+1)==rr.getNumColumns()) 
    {
      return a;
    }
  }
  return 0;
}
function NextBlankRange(Sheet, CurPos, FirtColumn, LastColumn)
{
  for (a=CurPos+1;a<Sheet.getLastRow()+1;a++)
  {
    var rr=Sheet.getRange(FirtColumn + a + ':' + LastColumn + a);
    var Rslt=rr.getValues().toString();
    if ((Rslt.length+1)==rr.getNumColumns()) 
    {
      return a;
    }
  }
  return 0;
}
function LastBlankRange(Sheet, FirtColumn, LastColumn)
{
  for (a=Sheet.getLastRow();a>1;a--)
  {
    var rr=Sheet.getRange(FirtColumn + a + ':' + LastColumn + a);
    var Rslt=rr.getValues().toString();
    if ((Rslt.length+1)==rr.getNumColumns()) 
    {
      return a;
    }
  }
  return 0;
}