Platforms/RaspberryPi: Fix NULL AssetTag in SMBIOS

Commit 6d4fed69 added support for
reporting AssetTag in RPi SMBIOS Types 2 and 3. The default
AssetTag is an empty string. SMBIOS does not allow empty strings to be
referenced from the corresponding string field. This caused breakage in
parsing SMBIOS Types 2 and 3 fields that follow the AssetTag field.

The issue caused an FWTS test failure, as reported in:
https://github.com/pftf/RPi4/issues/75



The fix is to detect if no AssetTag is set in the UEFI variable, and if
so, change the AssetTag SMBIOS field to an empty blank space.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Pete Batard <pete@akeo.ie>
Cc: Andrei Warkentin <awarkentin@vmware.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>