When an inbox (or folder) is empty, it might be nice to explicitly state this is the case in the message view area (in either Table or Cards View), similar to the message displayed in Outlook for example which says "We didn't find anything to show here" (see the attached image).
The Message List Header shows how many messages an inbox or folder contains, including '0' when there are no messages, but this can be overlooked or is not obvious enough, or might not even be visible if it is hidden.