Message of type systemMessageAgent can add redudant date message

[nlamah]

Describe the Bug
When a case on the backend is closed on a day that has no chats sent will result in a feed with a dateCell that has no message.

To Reproduce
See for more information the iOS equivalent of this issue. parley-messaging/ios-library#128

Expected behaviour
When there is no message for a certain date (agent, user, more?..) then the date should not be shown.

Screenshot
NB: This is a screenshot on iOS device