system: don't fail when Boot service isn't there

If the boot service isn't there, we shouldn't fail the response but omit
the Boot property.

Tested:
1. Service Validator passed on system without boot service
2. Service Validator passed on system with boot service

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Id3488f827736aa65d73e4a42082b279ca386ac3e