Fixes#6143
When doing post-image generation, we also write values for columns not
in delta (actual update), based on data selected in pre-image row.
However, if we are doing initial update/insert with only a subset of
columns, when the pre-image result set is nil, this cannot be done.
Adds check to non-touched column post-image code. Also uses the
pre-image value extractor to handle non-atomic sets properly.
Tests updated.
65a6ebbd73