invalid junit format on success case

[JanPaulBeumer]

Hi,
at first thanks for the awesome tool <3 and thanks for providing junit format.

i was using the junit format on my ci. there is a problem with no errors output, since it needs to have a testcase. otherwise its not valid junit format.

current output on success:
<?xml version="1.0" encoding="UTF-8"?><testsuites></testsuites>

how output should be:
<?xml version="1.0" encoding="UTF-8"?><testsuites><testsuite name="unknown classes" failures="0"><testcase name="All OK" assertions="1"/></testsuite></testsuites>

any other valid junit format is for sure fine aswell