Fixes https://github.com/payloadcms/payload/issues/11002 `buildVersionFields` was adding `null` version fields to the version fields array. When RenderVersionFieldsToDiff tried to render those, it threw an error. This PR ensures no `null` fields are added, as `RenderVersionFieldsToDiff` can't process them. That way, those fields are properly skipped, which is the intent of `modifiedOnly`