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
{
		return oa[iCursor++];
	}
	/**
	 * @return
	 */
	public final Calendar nextPrimitiveDateTime( )
	{
		cReused.setTimeInMillis( la[iCursor++] );
		return cReused;
	}
	/**
	 * 
	 * @return
	 */
	public boolean hasNext( )
	{
		if ( it != null )
		{
			return it.hasNext( );
		}
		else
		{
			return ( iCursor < iRowCount );
		}
	}
	/**
	 * 
	 * @return
	 */