Null-aware LeftMark joins miss logical NULLs in dictionary keys

[viirya]

Describe the bug

Null-aware LeftMark hash joins inspect the physical null bitmap of dictionary-encoded join keys instead of their logical nulls. A DictionaryArray key can be physically valid while pointing to a NULL dictionary value, so the join misses that NULL and produces an incorrect FALSE mark where SQL three-valued logic requires NULL.

The problem affects both sides of an uncorrelated null-aware mark join:

• A logical NULL build key is treated as non-null when the final mark column is built.
• A logical NULL probe key is not recorded in the shared probe summary, including when it is seen by a different probe partition from the partition that completes the join.

To Reproduce

At the HashJoinExec level, use dictionary keys whose key bitmap contains no nulls but whose dictionary values contain a null:

• Build values [1, NULL, 4], probe values [1, 2, 3]: the expected marks are [TRUE, NULL, FALSE], but the build-side logical null is marked FALSE.
• Build values [1, 4], probe values containing [1, NULL]: the expected marks are [TRUE, NULL], but the unmatched build row is marked FALSE.

Expected behavior

Null-aware LeftMark joins should use Arrow logical-null semantics for dictionary arrays on both build and probe sides, producing the same SQL results as non-dictionary arrays.