Bug 201017 TopN, TopPercent, BottomPercent filters will lead NPE when the values contains NULL[07]
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/script/NEvaluator.java
qualifiedRowCounter = 0;
		rowIdList = null;
		valueList = null;
		N = -1;
	}
	protected abstract Object doCompare( Object value1, Object value2 ) throws DataException;
}
/**
 * The class that provides "Top N" calculation service
 *
 */
class TopNEvaluator extends NEvaluator
{
	protected Object doCompare( Object value1, Object value2 ) throws DataException
	{
		return ScriptEvalUtil.evalConditionalExpr( value1, IConditionalExpression.OP_GT, value2, null);
	}
}
/**
 * The class that provides "Bottom N" calculation service
 *
 */
class BottomNEvaluator extends NEvaluator
{
	protected Object doCompare( Object value1, Object value2 ) throws DataException
	{
		return ScriptEvalUtil.evalConditionalExpr( value1, IConditionalExpression.OP_LT, value2, null);
	}
}