• Resolve issue where zero values (0, "", false) could not be scanned into pointer fields
• Ensure proper handling of both NULL values and actual zero values
• Update pointer field assignment logic in unmarshal/getValueInterface functions

Description

This PR fixes a critical issue in the getValueInterface function within sqlx where NULL values and zero values from the database were not being correctly scanned into destination struct fields.

The Problem

Previously, when unmarshaling database rows into Go structs:

• NULL values could not be properly assigned to basic non-pointer fields (e.g., string, int)
• Zero values (empty string "", number 0, false) had incorrect behavior when scanning into pointer fields
• The distinction between actual NULL values and intentional zero values was not maintained

The Solution

Enhanced the getValueInterface function in sqlx/orm.go to properly handle:

• Conversion of database NULL values to the zero value of destination basic types
• Correct allocation and assignment of zero values to pointer fields
• Proper distinction between NULL (sets pointer to nil) and zero values (allocates pointer to zero value)

Tests Added

Comprehensive test coverage added in sqlx/orm_test.go:

1. Pointer Field Handling

• TestUnmarshalRowsZeroValueStructPtr: Tests zero values and NULL values in mixed pointer/non-pointer structs
• TestUnmarshalRowsAllNullStructPtrFields: Tests all-NULL scenarios for pointer-only structs

2. SQL Null Types

• TestUnmarshalRowsWithSqlNullTypes: Tests all sql.Null types (String, Int64, Float64, Bool) with NULL values
• TestUnmarshalRowsSqlNullWithMixedData: Tests mixed basic types and sql.Null types
• TestUnmarshalRowsSqlNullTime: Specialized test for sql.NullTime type

3. Edge Cases and Zero Value Scenarios

• TestUnmarshalRowsSqlNullWithEmptyValues: Distinguishes between empty values and NULL values
• TestUnmarshalRowsSqlNullStringEmptyVsNull: Specifically tests empty string vs NULL string handling

Test Coverage

The new tests validate:

• ✅ NULL → zero value conversion for basic types
• ✅ NULL → nil for pointer fields
• ✅ Zero values → allocated pointers with correct values
• ✅ All sql.Null* types handle NULL correctly
• ✅ Mixed structs with both pointer and non-pointer fields
• ✅ Time types with sql.NullTime
• ✅ Empty strings vs NULL strings distinction

Changes

1. Core Fix: Refactored sqlx/orm.go::getValueInterface function
2. Tests: Added 7 comprehensive test functions with multiple test cases
3. Validation: All existing tests continue to pass

Impact

• Prevents data loss when scanning NULL values into structs
• Maintains backward compatibility with existing code
• Provides robust handling for database NULL semantics