Filter out nil addendum before constructing index. (#546)
Filter out nil addendum before constructing index. (#546)

In the PR where we added concurrency we used a fixed length array to store
addendum to preserve the ordering from the base image when constructing the
final index.  However, with `--platform=...` this list may be filtered, which
gives us `nil` entries in our addendum.

This filters `nil` entries prior to constructing the index.

Fixes: https://github.com/google/ko/issues/544
ko-build/ko,ko-build/ko