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
double n_value = -1;
			try
			{
				n_value = DataTypeUtil.toDouble( n_object ).doubleValue();
			}
			catch ( BirtException e )
			{
				
				throw new DataException(ResourceConstants.INVALID_TOP_BOTTOM_ARGUMENT, e);
			}
			
			
			if( n_percent )
			{
				if( n_value < 0 || n_value > 100)
					throw new DataException(ResourceConstants.INVALID_TOP_BOTTOM_PERCENT_ARGUMENT);
				N = (int)Math.round( n_value / 100 * filterPassController.getRowCount() );
			}
			else
			{
				if( n_value < 0 )
					throw new DataException(ResourceConstants.INVALID_TOP_BOTTOM_N_ARGUMENT);
				N = (int)n_value;
			}
		}
		
		
		Object value = ScriptEvalUtil.evalExpr( op_expr, cx, scope, "Filter", 0 );
		
		if ( filterPassController.getPassLevel( ) == FilterPassController.FIRST_PASS )