Bug 124629 Use Top/Bottom n in a field containing null value inside will cause DataException.
Description: Use Top/Bottom n in a field containing null value inside will cause DataException. Steps to reproduce: 1. New a data source and dataset 2. Add a filter on a field, set operator to top 5 3. Click Preview Result. Result: A BIRT exception occurred. Plug-in Provider:Eclipse.org Plug-in Name:BIRT Data Engine Plug-in ID:org.eclipse.birt.data Version:2.0.0 Error Code:data.engine.DataConversionError Error Message:An error happened in data type conversion. Can not convert the value of {NULL_VALUE_!@#$%^&} to Double type.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/script/NEvaluator.java
{
	private Object[] valueList; 
	private int[] rowIdList;
	private int firstPassRowNumberCounter = 0;
	private int secondPassRowNumberCounter = 0;
	private int qualifiedRowCounter = 0;
	
	
	private int N = -1;
	
	
	private boolean n_percent = false;
	
	
	private IScriptExpression op_expr;
	 
	private IScriptExpression n_expr;
	
	private FilterPassController filterPassController;
	/**
	 * Create a new instance to evaluate the top/bottom expression