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
clear( );
		ca = cdta;
		iDataType = IConstants.DATE_TIME;
		iContentType = IConstants.NON_PRIMITIVE_ARRAY;
		iRowCount = ca.length;
	}
	/**
	 * @return
	 */
	public final int getDataType( )
	{
		return iDataType;
	}
	/**
	 * @return current index
	 */
	public final int getIndex( )
	{
		return iCursor;
	}
}