[BugFix] Fix list partition pruning of multi-value partitions containing NULL (backport #78946)

[mergify]

Why I'm doing:

ListPartitionPruner prunes !=, NOT IN and IS NOT NULL by allPartitions - nullPartitions, but nullPartitions holds every partition whose value list contains NULL. A multi-value partition like VALUES IN ("beijing", NULL) is dropped as a whole and its non-null rows are lost. An equi join derives col IS NOT NULL on the join key, so JOIN ... ON f.city = d.city hits it too.

CREATE TABLE fact (k INT, city VARCHAR(20))
PARTITION BY LIST (city) (
    PARTITION pmix VALUES IN ('beijing', NULL),
    PARTITION psh  VALUES IN ('shanghai'));
INSERT INTO fact VALUES (1, NULL), (2, 'beijing'), (3, 'shanghai');

SELECT k, city FROM fact WHERE city IS NOT NULL;   -- beijing is lost
SELECT k, city FROM fact WHERE city != 'shanghai'; -- empty

What I'm doing:

Only drop NULL-only partitions for null-rejecting predicates: a partition is removed only if it is in nullPartitions and holds a single value (isSingleValuePartition), the same guard the literal removal already uses. External tables (no ListPartitionInfo) are unchanged since each partition holds one value. IS NULL and <=> NULL keep using the "contains NULL" set.

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5

---

This is an automatic backport of pull request #78946 done by [Mergify](https://mergify.com).