test: make sure we don't access an unbound variable

```
testsuite-60.sh[759]: + grep -q '(mount-monitor-dispatch) entered rate limit'
testsuite-60.sh[571]: + sleep 5
testsuite-60.sh[761]: ++ date -u +%s
testsuite-60.sh[571]: + [[ 1627037066 -le 1627037061 ]]
testsuite-60.sh[571]: /usr/lib/systemd/tests/testdata/units/testsuite-60.sh: line 41: entered_rl: unbound variable
```

(cherry picked from commit f571d9d5)