False positives on SC3028

[aep-sunlife]

Hi,

I am using POSIX safe code in my shell scripts, but ShellCheck insists on presenting some spurious warnings.

if [ "${HOSTNAME:-}" = 'beavis' ]; then
    export JAVA_HOME="/opt/jenkins/java/jdk-${JAVA_VERSION}"
fi

In scripts/provision-jenkins.sh line 1:
if [ "${HOSTNAME:-}" = 'beavis' ]; then
      ^-----------^ SC3028 (warning): In POSIX sh, HOSTNAME is undefined.

ShellCheck should recognize that we're not hard depending on the existence of the HOSTNAME environment variable, but in fact falling back on POSIX's default value syntax. So no warning should emit.