When there is no message from the underlying failure exception e need…

… to substitute String.Empty for something else meaningful.

src/allure-nunit/Core/AllureNUnitHelper.cs

@@ -252,7 +252,9 @@ private void StopTestCase()
 
             AllureLifecycle.UpdateTestCase(x => x.statusDetails = new StatusDetails
             {
-                message = TestContext.CurrentContext.Result.Message,
+                message = string.IsNullOrWhiteSpace(TestContext.CurrentContext.Result.Message)
+                ? TestContext.CurrentContext.Test.Name 
+                : TestContext.CurrentContext.Result.Message,
                 trace = TestContext.CurrentContext.Result.StackTrace
             });