Skip writing empty SPL string into update manifest am: da0701854d am: c9fb0c4e85

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2204081

Change-Id: Ie1923e3b06b3a9bf5b7a522260175b80b0e76dfc
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>