Bug 145274 __isnull does not work properly when parameters have default value
steps to reproduce: - create a report - add a parameter with default value - preview report in web viewer - modify the url with __null pattern expected behavior: - __null takes effect actual behavior: - it simply didn't
viewer/org.eclipse.birt.report.viewer/birt/WEB-INF/classes/org/eclipse/birt/report/context/ViewerAttributeBean.java
String defalutValue = null;
		Object defaultValueObj = null;
		
		try
		{
			defaultValueObj = this.getReportService( )
					.getParameterDefaultValue( design, paramName, options );
		}
		catch ( ReportServiceException e )
		{
			e.printStackTrace( );
		}
		
		ScalarParameterHandle parameter = (ScalarParameterHandle) findParameter( paramName );
		
		if ( defaultValueObj != null && parameter != null )
		{
			defalutValue = ParameterValidationUtil.getDisplayValue( null,
					parameter.getPattern( ), defaultValueObj, locale );
		}
		
		if ( defalutValue == null && parameter != null )
		{
			defalutValue = parameter.getDefaultValue( );
		}
		return defalutValue;
	}
	/**
	 * if parameter existed in config file, return the correct parameter name
	 *