muon-updater fails to produce nginx config file replacement prompt

[Codename: Steeve Knight]

Symptom: muon-updater UI hangs on "preparing nginx-common" - reproducible

$ tail /var/log/dist-upgrade/apt-term.log -n 10

produces:

```
Configuration file '/etc/nginx/sites-available/default'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** default (Y/I/N/O/D/Z) [default=N] ?
```

but this is never produced on the UI for interaction