Correctly compare expressions with NULL and MISSING during index selection

See https://forums.couchbase.com/t/secondary-index-entries-for-docs-with-missing-key-values/9183/5

During index selection, we compare the expression in the index key with the expression in the query predicate for equivalence. For this comparison, NULL should compare as equivalent to NULL, and MISSING should compare as equivalent to MISSING.

Because the comparison is not happening correctly, the user is unable to use an index on IFMISSING(field, NULL).