LPS-40873 Poller: allow empty mails array

In fact, it wasn't like this for nothing. I prefered having an empty mails array to signify "no mails" to the front-end than having no mails array at all. This simplifies the front-end (and doesn't really complexify the back-end) since it will just iterate all received mails instead of checking if the property exists, and then iterating if so.

Tell me if you agree.