Passing the NULL enumerator to g_file_enumerator_next_file makes
us overwrite error information otherwise:

    $ sudo bin/virt-sandbox-service create --unitfile foo.service foo
    (process:18529): GLib-GIO-CRITICAL **: g_file_enumerator_next_file: assertion 'G_IS_FILE_ENUMERATOR (enumerator)' failed

    (process:18529): GLib-GIO-CRITICAL **: g_file_enumerator_close: assertion 'G_IS_FILE_ENUMERATOR (enumerator)' failed
    bin/virt-sandbox-service:318: Warning: GError set over the top of a previous GError or uninitialized memory.
    This indicates a bug in someone's code. You must ensure an error is NULL before it's set.
    The overwriting error message was: Error removing file: No such file or directory context.undefine()
    bin/virt-sandbox-service:318: Warning: g_object_unref: assertion 'G_IS_OBJECT (object)' failed context.undefine()
    Cleanup failed: g-io-error-quark: No such file or directory (1)
    bin/virt-sandbox-service: g-io-error-quark: No such file or directory (1)

also make sure we don't unref NULL in this case.