package should handle empty dbfile better

[timeless]

I performed a `do-release-upgrade` from `bionic` to `focal` and had `roundcube` partially configured w/ `mysql` (or maria or whatever, dunno, it was ages ago).

Some excerpts from `/var/log/dist-upgrade/screenlog.0`:

```
Unpacking dbconfig-common (2.0.9) over (2.0.4ubuntu1) ...
```

```
creating database backup in /var/cache/dbconfig-common/backups/roundcube_1.2~beta+dfsg.1-0ubuntu1.2019-03-05-21.12.19.
applying upgrade sql for 1.2~beta+dfsg.1-0ubuntu1 -> 1.2.0-1.
error encountered processing /usr/share/dbconfig-common/data/roundcube/upgrade/mysql/1.2.0-1:
mysql said: ERROR 1060 (42S21) at line 2: Duplicate column name 'failed_login'
An error occurred while upgrading the database:

mysql said: ERROR 1060 (42S21) at line 2: Duplicate column name 'failed_login'
```

```
dbconfig-common: writing config to /etc/dbconfig-common/roundcube.conf
applying upgrade sql for 1.3.6+dfsg.1-1 -> 1.4.1-1.
error encountered processing /usr/share/dbconfig-common/data/roundcube/upgrade/mysql/1.4.1-1:
mysql said: ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)
ESC[?1049hESC[?1hESC=ESC[1;25rESC[4lESC[?25lESC[mESC[37mESC[40mESC[1;25rESC[HESC[JESC[1;1HESC[1mESC[37mESC[45m
```

Here's what I saw on my screen:

```
Package configuration

  ┌────────────────────────────────────────────────────────────────────────┤ Configuring roundcube-core ├─────────────────────────────────────────────────────────────────────────┐
  │ │
  │ An error occurred while upgrading the database:
  │
  │ mysql said: ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)
  │
  │ Fortunately, should hold a backup of the database, made just before the upgrade (unless the error occurred during backup creation, in which case no changes will have been
  │ applied yet). Your options are:
  │ * abort - Causes the operation to fail; you will need to downgrade,
  │ reinstall, reconfigure this package, or otherwise manually intervene
  │ to continue using it. This will usually also impact your ability to
  │ install other packages until the installation failure is resolved.
  │ * retry - Prompts once more with all the configuration questions
  │ (including ones you may have missed due to the debconf priority
  │ setting) and makes another attempt at performing the operation.
  │ * retry (skip questions) - Immediately attempts the operation again,
  │ skipping all questions. This is normally useful only if you have
  │
  │ <Ok>
  │ │
  └───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
```

According to:
https://github.com/deepin-community/dbconfig-common/blob/792a7961fb442be3e8aed39cc2b1f93a3073b8a6/debian/dbconfig-common.templates#L175

the relevant text is:
```
Fortunately, ${dbfile} should hold a backup of the database, made just before
```

Whatever part of `dbconfig-common` is responsible for displaying this text should confirm it has a `${dbfile}` before it goes off and displays this text (or perhaps before it goes down the path that leads to displaying this text).

My current version is:
dbconfig-common 2.0.22~pgdg18.04+1

```
# lsb_release -rd
Description: Ubuntu 20.04.5 LTS
Release: 20.04
```

```
# apt-cache policy dbconfig-common
dbconfig-common:
  Installed: 2.0.22~pgdg18.04+1
  Candidate: 2.0.22~pgdg18.04+1
  Version table:
 *** 2.0.22~pgdg18.04+1 100
        100 /var/lib/dpkg/status
     2.0.13 500
        500 http://us-east1.gce.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
```

(But note that this may not have been the current version at the time of the instance... it's a bit hard for me to tell from the logs I have. That said, I doubt that the code has significantly changed.)

Expected results
1. Ensure strings that should have values have values before using them
2. Include quotes around such strings (even seeing ` "" ` is better than seeing ` ` as it hints that there's a problem)

Actual results
```
Fortunately, should hold a backup of the database, made just before the upgrade (unless the error occurred during backup creation, in which case no changes will have been
applied yet). Your options are:
```

[Seth Arnold (seth-arnold)]

Weird, launchpad ate the tell-tale double spaces!

"Fortunately, should hold a backup of the database"
originally looked like:
"Fortunately, should hold a backup of the database"

(two spaces after ","). timeless shared the pastebins with me while fighting this.

[Seth Arnold (seth-arnold)]

Heh, it looks like it ate the double-spaces in my comment, too.