Bug 182673 No default value error message prompts incorrectly
Steps to reproduce: 1. New a report parameter; 2. Choose control type as Combo; 3. Choose parameter as dynamic type; Expected result: As the isRequired is false by default, parameter's default value can be null value. There should be no error message about no default value prompt. Actual result; Error messages prompts
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/CascadingParametersDialog.java
for ( Iterator iter = getColumnList( ).iterator( ); iter.hasNext( ); )
			{
				ResultSetColumnHandle column = (ResultSetColumnHandle) iter.next( );
				String columnName = column.getColumnName( );
				if ( DEUtil.getColumnExpression( columnName )
						.equalsIgnoreCase( selectedParameter.getValueExpr( ) ) )
				{
					queryExpr = DEUtil.getResultSetColumnExpression( columnName );
					break;
				}
			}
			if ( queryExpr == null || queryExpr.equals( "" ) )
			{
				return Collections.EMPTY_LIST;
			}
			ScriptExpression expression = new ScriptExpression( queryExpr );
			String columnBindingName = "_$_COLUMNBINDINGNAME_$_";
			query.addResultSetExpression( columnBindingName, expression );
			
			IPreparedQuery preparedQuery = engine.prepare( (IQueryDefinition) query );
			IQueryResults results = preparedQuery.execute( null );
			if ( results != null )
			{
				IResultIterator iter = results.getResultIterator( );
				if ( iter != null )
				{
					DateFormatter formatter = new DateFormatter( STANDARD_DATE_TIME_PATTERN,
							ULocale.US );
					while ( iter.next( ) )