trivial: Allow using NULL as func for fu_device_add_possible_plugin()
trivial: Allow using NULL as func for fu_device_add_possible_plugin()

This should be used when a device recovery is not possible.

fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd