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
public int size( )
	{
		return iRowCount;
	}
	/**
	 * 
	 */
	public final void reset( )
	{
		iCursor = 0;
		if ( co != null )
		{
			it = co.iterator( );
		}
	}
	/**
	 * 
	 * @return
	 */
	public final Object first( )
	{
		reset( );
		return next( );
	}
	/**
	 * 
	 * @return
	 */
	public final Object last( )
	{