setup-interfaces: do not touch existing setup if user chooses "done"

In interactive mode, user may not want to alter settings and will enter "done" on first question (typical on headless setup, with working network settings).
Currently, hitting "done" overwrites eventually existing /etc/network/interfaces with default loopback entry.
Restarting interfaces then just kills the existing connection and abort all setup process.

Can we keep (pre)existing /etc/network/interfaces if user select done right-away on first question?
Thanks for consideration.