Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/computation/DataSetIterator.java
iDataType = IConstants.DATE_TIME;
		iContentType = IConstants.NON_PRIMITIVE_ARRAY;
		iRowCount = ca.length;
		updateDateTimeValues( );
	}
	/**
	 * 
	 * @param ds
	 * @throws UnexpectedInputException
	 */
	public DataSetIterator( Object oContent, int iDataType )
			throws IllegalArgumentException, ChartException
	{
		if ( ( iDataType & IConstants.LINEAR ) == IConstants.LINEAR
				|| ( iDataType & IConstants.LOGARITHMIC ) == IConstants.LOGARITHMIC )
		{
			iDataType = IConstants.NUMERICAL;
		}
		this.iDataType = iDataType;
		if ( iDataType == IConstants.NUMERICAL )
		{
			if ( oContent instanceof Collection )
			{
				iContentType = IConstants.COLLECTION;
				co = (Collection) oContent;
			}
			else if ( oContent instanceof double[] )
			{
				iContentType = IConstants.PRIMITIVE_ARRAY;
				da = (double[]) oContent;