[fix](json) avoid json_set crash on invalid jsonb value

[mrhhsg]

What problem does this PR solve?

Issue Number: None

Problem Summary: json_set may receive an invalid JSONB payload from a nullable expression and crash when converting it to rapidjson. This patch treats invalid JSONB payloads as JSON null in the json modify value path and uses the correct row index for nullable constant value columns.

Release note

None

Check List (For Author)

• Test:
  • Build: doris-local-regression --network 10.26.20.3/24 --offset 42300 build
  • Regression test: doris-local-regression --network 10.26.20.3/24 --offset 42300 run -d nereids_p0/sql_functions/json_function -s test_query_json_set_jsonb_nullable_value
  • git diff --check
• Behavior changed: No
• Does this need documentation: No

[mrhhsg]

/review

[github-actions]

Review opinion: the PR looks safe to proceed. I did not find any critical blocking issues in the changed backend logic or regression test.

Critical checkpoints:

• Correctness: The nullable constant index fix uses the same indexed row for both the value column and its null map, which addresses the likely out-of-bounds/crash path. The JSONB null fallback also prevents dereferencing a null JsonbValue.
• Scope/regression risk: The JSONB parser fallback is global to the JSON function value parser, but it maps unparsable JSONB payloads to JSON null rather than crashing; I did not find a concrete incompatible behavior regression in this PR context.
• Tests: The new regression covers nullable JSONB values and the invalid JSONB crash scenario with enough rows to exercise vectorized row/null-map handling. git diff --check passed locally.
• Documentation/compatibility: No user-facing syntax or documented behavior change found that requires docs.
• User focus: No additional user-provided review focus was present.

[mrhhsg]

run buildall