When a column store index is limited to a subset of columns (either by a columnstoreProjection or by "foo.$**"-style notation), it is possible for a document replacement update to get forwarded to the index even when it does not modify any of the indexed columns. When this happens during bulk building, it results in a "side write" that has no entries.

That behavior is semantically correct but trips an invariant when it eventually gets processed. There is no need for these no-op entries, though, and it is a simple to change the index update code so that it does not write them.