Check if attachments object is empty string (elsa-workflows#3164)

If attachments object is empty string, ignore and return from method.

src/activities/Elsa.Activities.Email/Activities/SendEmail/SendEmail.cs

@@ -121,6 +121,9 @@ private async Task AddAttachmentsAsync(BodyBuilder bodyBuilder, CancellationToke
 
             if (attachments != null)
             {
+                if (attachments is string && string.IsNullOrWhiteSpace((string)attachments))
+                    return;
+
                 var index = 0;
                 var attachmentObjects = InterpretAttachmentsModel(attachments);
 
@@ -206,4 +209,4 @@ private async Task<HttpResponseMessage> DownloadUrlAsync(Uri url)
             return response;
         }
     }
-}
+}