[efi] Provide dummy device path in efi_image_probe()

Some UEFI platforms will fail the call to LoadImage() with
EFI_INVALID_PARAMETER if we do not provide a device path (even though
we are providing a non-NULL SourceBuffer).

Fix by providing an empty device path for the call to LoadImage() in
efi_image_probe().

The call to LoadImage() in efi_image_exec() already constructs and
provides a device path (based on the most recently opened SNP device),
and so does not require this fix.

Reported-by: NICOLAS CATTIE <nicolas.cattie@mpsa.com>
Tested-by: NICOLAS CATTIE <nicolas.cattie@mpsa.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>