feat: impl function strip_null_value for Variant

[KKould]

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Summary

close #17627

-- Create a simple table with JSON data
CREATE TABLE users (user_data VARIANT);

-- Insert two records: one with a null phone number, one with a value
INSERT INTO users 
  SELECT PARSE_JSON('{"name": "Alice", "phone": null}');
  
INSERT INTO users 
  SELECT PARSE_JSON('{"name": "Bob", "phone": "555-1234"}');

-- Compare regular access vs using STRIP_NULL_VALUE
SELECT 
  user_data:name AS name,
  user_data:phone AS original_phone,      -- JSON null appears as 'null'
  STRIP_NULL_VALUE(user_data:phone) AS phone_number  -- Converts JSON null to SQL NULL
FROM users;

-- Result:
-- +----------+----------------+--------------+
-- | NAME     | ORIGINAL_PHONE | PHONE_NUMBER |
-- +----------+----------------+--------------+
-- | "Alice"  | null           | NULL         |
-- | "Bob"    | "555-1234"     | "555-1234"   |
-- +----------+----------------+--------------+

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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This change is