`/etc/default/tomcat9` resets existing `JAVA_OPTS`

[David Bouman]

Ubuntu Release:

> Description: Ubuntu 22.04.3 LTS
> Release: 22.04

Package version:

> tomcat9:
> Installed: 9.0.58-1ubuntu0.1
> Candidate: 9.0.58-1ubuntu0.1
> Version table:
> *** 9.0.58-1ubuntu0.1 500
> 500 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu jammy-updates/universe amd64 Packages
> 100 /var/lib/dpkg/status
> 9.0.58-1 500
> 500 http://eu-west-1.ec2.archive.ubuntu.com/ubuntu jammy/universe amd64 Packages

Offending section from `/etc/default/tomcat9`:

```sh
# You may pass JVM startup parameters to Java here. If you run Tomcat with
# Java 8 instead of 9 or newer, add "-XX:+UseG1GC" to select a suitable GC.
# If unset, the default options will be: -Djava.awt.headless=true
JAVA_OPTS="-Djava.awt.headless=true"
```

Particularly, the comment states _"If unset, the default options will be: `-Djava.awt.headless=true`"._

However, the code doesn't do that; it just disregards any previous value that `JAVA_OPTS` might have had.

Rather, to reflect the intent, the code should do something along the lines of:

```sh
if [ -z "$JAVA_OPTS" ] ; then
    JAVA_OPTS="-Djava.awt.headless=true"
fi
```

Why is this a problem?

If a systemd unit drop-in (`/etc/systemd/system/tomcat9.service.d/override.conf`) wants to initialize `JAVA_OPTS` to a different value, that setting will currently be lost because `/etc/default/tomcat9` runs after this.

Moreover, the main `/lib/systemd/system/tomcat9.service` _also_ initializes `JAVA_OPTS`, even though it is just as futile here. I'm only guessing that this went unnoticed because it simply happens to use the same default value as `/etc/default/tomcat9`.