create_snapshot: Generate "no-op" patches based only on partition size

This CL gives create_snapshot the abiliy to generate "no-op" patch files
when there are no source or target partition images available and the
only known information is the target partition size in bytes (which can
be specified via a new --partition_size flag). Here, a "no-op" patch
is essentially an empty patch with no operations, which effectively
leaves the target partition intact.

Motivation: Unlock the ability to update only a subset of the partitions
in the super partition. This is useful during kernel development, where
rebuilding and flashing only the kernel-related partitions is often much
faster than rebuilding and flashing the whole OS.

Context: I'm making changes to deflash.py to support the aforementioned
use case. See http://ag/33534126 and the attached bug for more context.

Additional context: Applying patch files for only a subset of the
partitions within super (e.g. via "snapshotcl apply-updates") and then
rebooting causes snapuserd to fail to mount those partitions within
super for which no patch files were provided. This causes the system to
reboot, initiating a boot loop. As a workaround, no-op patches seem to
appease snapuserd while achieving the desired result.

Test: Flash a device with deflash.py and http://ag/33534126
Bug: 408522717

Change-Id: I8eef9edd2b5767aee0f027f675ae623215d70e06