Fix ZERO and DISCARD operations.

These operations were broken (and not used in the generator) because
the code expects them to have a blob offset, but they don't. This bug
prevents them from being used in full payloads, but can still be used
in delta payloads.

Bug: 27858697
TEST=Deployed a full payload with ZERO operations to the new update_engine.

Change-Id: Ifc67bd7c3b2a81593d0347a345723cc34eaaf6b5