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
return null;
		}
		
		instance.op_expr = op_expr;
		instance.n_expr = n_expr;
		instance.filterPassController = filterPassController;
		return instance;
	}
	
	
	/**
	 * Evaluate the given value
	 * @param value
	 * @param n
	 * @return
	 * @throws DataException
	 */
	public boolean evaluate( Context cx, Scriptable scope ) throws DataException
	{
		if( filterPassController.getForceReset() )
		{
			doReset();
			filterPassController.setForceReset( false );
		}
		
		if ( N == -1 )
		{
			
			
			Object n_object = ScriptEvalUtil.evalExpr( n_expr, cx, scope, "Filter", 0 );