TestSuiteStartedEvent.Name gets a default value.

When generating an xml with [AllowEmptySuites](https://github.com/allure-framework/allure-csharp-commons/blob/master/AllureCSharpCommons/AllureConfig.cs#L11) set to true, the resulting xml doesn't have a 'name' element which is [required by the allure schema](https://github.com/allure-framework/allure-core/blob/master/allure-model/src/main/resources/allure.xsd#L11).

I've added a default value of String.Empty so the resulting xml after serialization will have a 'name' element and pass the schema validation.

Note: the resulting XML without the change still generates a valid allure report, it just doesn't pass the schema validation.