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
* @param operator 
	 * @param op_expr operand expression
	 * @param n_expr expression to yield N 
	 * @return
	 */
	public static NEvaluator newInstance( int operator, IScriptExpression op_expr, 
			IScriptExpression n_expr, FilterPassController filterPassController  )
	 	throws DataException
	{
		NEvaluator instance = null;
		switch ( operator )
		{
			case IConditionalExpression.OP_TOP_N :
				instance = new TopNEvaluator();
				instance.n_percent = false;
				break;
			case IConditionalExpression.OP_TOP_PERCENT:
				instance = new TopNEvaluator( );
				instance.n_percent = true;
				break;
			case IConditionalExpression.OP_BOTTOM_N:
				instance = new BottomNEvaluator( );
				instance.n_percent = false;
				break;
			case IConditionalExpression.OP_BOTTOM_PERCENT:
				instance = new BottomNEvaluator( );
				instance.n_percent = true;
				break;
			default:
				assert false;