fix: stmt.Changed zero value filed behavior

[a631807682]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Statement.Changed() behavior should be same when using map and struct.

// tx.Statement.Changed("Code") => true
DB.Model(&product).Select("Name", "Code", "Price").Updates(map[string]interface{}{"Name": "Product New4", "code": ""})
// tx.Statement.Changed("Code") => false
DB.Model(&product).Select("Name", "Code", "Price").Updates(Product3{Name: "Product New4", Code: ""})

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