Allow creation of a logical partition

- InstallPlan: allow source partition to be missing if size == 0
- DeltaPerformer: don't open source partition if size == 0

Also:
- DeltaPerformer: test for source_fd_ before using it
  (to avoid segfault)
- Fix DeltaPerformerTest to generate payloads with correct
  source sizes and paths. Previously, this works because source_path
  is always set even when source_size = 0.

Bug: 124958572
Test: python system/update_engine/scripts/update_device.py <ota-img>
Test: update_engine_unittests

Change-Id: I53bda1d12accdf91e7fa616be5da562e189038d5