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
{
			displayTypeChooser.clearSelection( ); 
		}
		else
		{
			displayTypeChooser.setText( getInputDisplayName( ) );
		}
		defaultValue = selectedParameter.getDefaultValue( );
		if ( getSelectedDataType( ).equals( DesignChoiceConstants.PARAM_TYPE_STRING ) )
		{
			if ( defaultValue == null )
			{
				defaultValueChooser.setText( CHOICE_NULL_VALUE );
			}
			else if ( defaultValue.equals( "" ) )
			{
				defaultValueChooser.setText( CHOICE_BLANK_VALUE );
			}
			else
			{
				defaultValueChooser.setText( defaultValue );
			}
		}
		else
		{
			if ( defaultValue == null )
			{
				defaultValueChooser.setText( "" ); 			
			}
			else