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_!@#$%^&} to Double type.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/script/NEvaluator.java
* @throws DataException
	 */
	private void populateValueListAndRowIdList( Object value, int N ) throws DataException
	{
		assert N>=0;
		for( int i = 0; i < N; i++ )
		{
			if( value == null )
				value = "{NULL_VALUE_!@#$%^&}";
			if( valueList[i] == null )
			{
				valueList[i] = value;
				rowIdList[i] = firstPassRowNumberCounter;
				break;
			}else
			{
				Object result = this.doCompare( value, valueList[i] );
				try
				{
					
					if ( DataTypeUtil.toBoolean( result ).booleanValue( ) == true )
					{
						for( int j = N - 1; j > i; j--)
						{
							valueList[j] = valueList[j-1];
							rowIdList[j] = rowIdList[j-1];
						}
						valueList[i] = value;
						rowIdList[i] = firstPassRowNumberCounter;
						break;