/etc/network/interfaces is almost empty

This is normal. What you have is exactly the same as what's in /etc/network/interfaces on my Ubuntu 14.04 system (which is working fine and networking actively through two separate physical interfaces).

By default an Ubuntu desktop system uses NetworkManager to manage network connections automatically and dynamically. This does not require (and in fact typically requires there not be) entries detailing configuration of your physical interfaces in /etc/network/interfaces.

(NetworkManager, if changed from running in "unnmanaged mode" to "managed mode," will recognize and work with /etc/network/interfaces entries. But unmanaged mode is the default.)

Sources:

• Ubuntu help wiki - Network Manager - Issues
• nux's answer to How do I use /etc/network/interfaces instead of network-manager
• NetworkManager (on Debian wiki)
• NetworkManager's README.Debian file (also /usr/share/doc/network-manager/README.Debian, and related)
• BatchyX's answer to Wireless Networks showing “device not managed” in Network Manager