Add call to action to private message empty screen

Contrary to other empty view screens, this is one that a lot of new users on the platform will encounter.

Improve the private message screen by changing the empty text to "You do not have any private messages yet. Click on the button on the right to start a new chat."

A client suggested "Hi there! You don't have any private messages yet. Click on the button on the right to start a new chat." but this may not be appropriate tone of voice for all platforms.