Approver cannot approve empty output

[anuratransfers]

For example, given the code:

@Rule @JvmField val approver = ApprovalsRule.usualRule()!!

@Test fun reads_excluded_journal_ids_from_resources() {
    val contexts = HiddenContexts()
    val stableSet = contexts.journalSuggesterExcludedJournalIds.map { Integer.parseInt(it) }.toSortedSet()
    approver.assertApproved(stableSet.joinToString("\n"))
}

Empty stableSet can never be approved because internally LazyOutputStream will not create .actual file and will compare empty stableSet to null.