Implement wasNull tracking in ResultSet getter methods

[mingseok]

Summary

Implemented comprehensive wasNull tracking and refactored getter methods in JDBC4ResultSet to ensure JDBC specification compliance and improve code maintainability.

Changes

Added wasNull tracking to all getter methods: Covers primitive types, objects, dates/times, streams, and BigDecimal
Refactored columnLabel getters to use delegation pattern: Eliminates code duplication and ensures consistent wasNull behavior

Bug Fixes & Code Quality

• Fixed getString(String) to return null instead of empty string for null values
• Added @nullable annotation to getBytes(String) to fix NullAway error
• Preserved String parsing in getDate(String) for TEXT-formatted dates
• Extracted timezone offset calculation to helper method

Testing

Added comprehensive tests for wasNull tracking, columnLabel getters, stream methods, and null handling