refactor(email): replace Task.WhenAll with foreach in SendEmailBehavior

Changed the SendFinalEmailToReceiversAsync method to use a sequential `foreach` loop
instead of `Task.WhenAll` for sending history commands. Added a null check for
receiver emails to prevent potential exceptions.

EnvelopeGenerator.Finalizer/appsettings.Development.json

@@ -1,7 +1,7 @@
 {
   "Logging": {
     "LogLevel": {
-      "Default": "Information",
+      "Default": "Error",
       "Microsoft.Hosting.Lifetime": "Information"
     }
   },