On one machine (this may be hardware-dependent; the system in question is a
Dell Precision 3450 with BIOS version 1.2.3) fwupd logs the message

GLib g_bytes_get_data: assertion 'bytes != NULL' failed

exactly 16 times, twice a day (coinciding with activations of
fwupd-refresh.service, regardless of whether new metadata is available).

I guess this means that some calls to g_bytes_get_data() are missing a guard
against the possibility that the first argument is null. (The second argument
is allowed to be null according to the documentation and isn't called "bytes".)