Don't write vendor to dynamic_partitions_info.txt if it's unavailable.

If an AOSP target is built with dynamic partitions support but without
vendor.img available at build time, don't write 'vendor' to
dynamic_partitions_info.txt that will be used for building OTA packages.

We can't remove the partition from BoardConfig file, because the name
will be still needed in other places, such as generating the partition
metadata (super_empty.img). Otherwise manually flashing vendor.img at a
later point would require additional steps (e.g. `fastboot
create-logical-partition`).

Bug: 120852744
Test: `m -j dist` with and without vendor projects.
Change-Id: Ia1c3ed5bbea0255f1908958140dac97a1597586d