Variant shredding spec ambiguously refers to both SQL NULL and Variant null as "null"

### Describe the enhancement requested

The current variant shredding spec wording is confusing, because it's not always easy to tell whether a `value` is Variant null or missing (SQL NULL). It would improve readability if the two concepts were more clearly separated.

Start by reading the current variant shredding specification and locate passages that use “null” for both Variant null and missing SQL NULL values. Clarify the terminology consistently throughout the spec, then verify that each reference clearly distinguishes the two concepts.

Written by the indexing model from the issue text.