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
{
			choiceSet = DEUtil.getMetaDataDictionary( )
					.getChoiceSet( DesignChoiceConstants.CHOICE_STRING_FORMAT_TYPE );
		}
		else if ( DesignChoiceConstants.PARAM_TYPE_DATETIME.equals( type ) )
		{
			choiceSet = DEUtil.getMetaDataDictionary( )
					.getChoiceSet( DesignChoiceConstants.CHOICE_DATETIME_FORMAT_TYPE );
		}
		else if ( DesignChoiceConstants.PARAM_TYPE_DECIMAL.equals( type )
				|| DesignChoiceConstants.PARAM_TYPE_FLOAT.equals( type )
				|| DesignChoiceConstants.PARAM_TYPE_INTEGER.equals( type ) )
		{
			choiceSet = DEUtil.getMetaDataDictionary( )
					.getChoiceSet( DesignChoiceConstants.CHOICE_NUMBER_FORMAT_TYPE );
		}
		return choiceSet;
	}
	private String getSelectedDataType( )
	{
		String type = null;
		if ( StringUtil.isBlank( dataTypeChooser.getText( ) ) )
		{
			if ( selectedParameter != null )
			{
				type = selectedParameter.getDataType( );
			}
			else
			{
				type = DesignChoiceConstants.PARAM_TYPE_STRING;