journal: skip part of test-journal-interleaving if no machine-id exists

journal: skip part of test-journal-interleaving if no machine-id exists

When executed on a systemd with an empty /etc/machine-id,
test-journal-interleaving fails in test_sequence_numbers_one() when
re-opening the existing "two.journal". This is because opening the
existing journal file with managed_journal_file_open() causes
journal_file_verify_header() to be called. This function tries to
compare the current machine-id to the machine-id in the journal file
header, but does not handle the case where the machine-id is empty or
non-existent.

Check if we have an initialized machine-id before executing this portion
of the test.