"Unique scan" is incorrectly reported in the explained plan for unique index and IS NULL predicate

[sim1984]

SELECT *
FROM RDB$RELATIONS
WHERE RDB$RELATION_NAME IS NULL

Explain plan

Select Expression
    -> Filter
        -> Table "RDB$RELATIONS" Access By ID
            -> Bitmap
                -> Index "RDB$INDEX_0" Unique Scan

Unique indexes can generally contain multiple NULL values, so "Unique scan" is only possible for pure equality (=) comparisons, but not for IS NULL and IS NOT DISTINCT FROM predicates. For such predicates, "Range Scan (full match)" is expected.

Select Expression
    -> Filter
        -> Table "RDB$RELATIONS" Access By ID
            -> Bitmap
                -> Index "RDB$INDEX_0" Range Scan (full match)