stub: NULL checks for DeviceHandle and FilePath

stub: NULL checks for DeviceHandle and FilePath

UKIs may be loaded in a way, that there can not be a device handle to
the filesystem, that contains the image, for example when using a
bootloader to load the image from a partition with a file system that is
not supported by the firmware.

With the current systemd stub, this causes a failed assertion, because
stub gets passed a NULL DeviceHandle and FilePath. Inserting two
explicit checks enables proper boot even in this case.

Fixes: #29331