Fix Bash4 behavior with set -o nounset

[mikynov]

This PR fixes inconsistent behavior between Bash 4.4 and lower versions while using set -o nounset.

I believe set -o nounset is very helpful for more complex runnerfiles and this PR should address two things:

1. Unbound variables in runner_bootstrap and runner_run_task
2. Empty task name in runner_bootstrap when called without parameters (i.e. default task should be invoked)

Example runnerfile.sh:

#!/usr/bin/env bash
cd "$(dirname "$0")" || exit
source src/runner.sh

set -o xtrace
set -o errtrace
set -o nounset
set -o pipefail

task_default() {
    runner_log "I'm default task: ${*}"
}

task_example() {
    runner_log "I'm example task: ${*}"
}

Using Bash version prior to 4.4 the runnerfile.sh was failing with either "unbound variable" or "Task '' is not defined!".

Tested in Bash 4.0, 4.1, 4.2, 4.3, 4.4 and 5.0 from https://hub.docker.com/_/bash.

Please consider merging this because Bash versions < 4.4 are still pretty common.

[stylemistake]

Looks a bit hacky. Could it all be fixed by properly initializing those vars?

[mikynov]

Fine, I should have done better investigation...

I've found the SO answer about this, containing also reasoning and link to Bash changelog. It looks like "unbound variable" error is expected behavior in Bash < 4.4 while referencing an empty array with arr[@].

Suggested workaround looks "hacky" as well but works better:

Use ${arr[@]+"${arr[@]}"} instead of "${arr[@]}".

[stylemistake]

Oof. Well, I guess I'll merge it like that.