fix(query): fix variant get string function auto cast null to SQL NULL

[b41sh]

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Summary

Variant get string function automatically converts json null to SQL NULL to avoid cast failures caused by null values.
for example:

root@0.0.0.0:48000/default> select cast(parse_json('{"key1":null}')->>'key1' as integer);
┌───────────────────────────────────────────────────────┐
│ CAST(parse_json('{"key1":null}') ->> 'key1' AS Int32) │
│                    Nullable(Int32)                    │
├───────────────────────────────────────────────────────┤
│                                                  NULL │
└───────────────────────────────────────────────────────┘
1 row read in 0.024 sec. Processed 1 row, 1 B (41.67 rows/s, 41 B/s)

• fixes: bug: JSON not evaluating null #17433

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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