Skip verifying the partition with no changed blocks.

For incremental BBOTAs, commit [1] changed to verify only the touched
blocks to reduce the update time. This CL fixes the bug when there's no
changed block between the two builds.

[1]: commit d522bdc9

Bug: 27813356
Bug: 28195498
Change-Id: Ia752d3cf034c544517458ed15df509d4596e21fa
(cherry picked from commit f9efe284)