fix: failed to modify field comment when empty due to incorrect conditions

[iTanken]

• [x] Do only one thing
• [x] Non breaking API changes
  • GetColumnComment(*gorm.Statement, string) string => GetColumnComment(*gorm.Statement, string) sql.NullString
• [x] Tested

What did this pull request do?

@jinzhu Fix issue where modifying field comments failed when the field comment was an empty string due to incorrect judgment conditions.

User Case Description

type TestTableFieldComment struct {
	ID       string     `gorm:"column:id;primaryKey;comment:"`  // field comment is an empty string
	// ...
}

func (*TestTableFieldComment) TableName() string { return "test_table_field_comment" }

// =>

type TestTableFieldCommentUpdate struct {
	ID       string     `gorm:"column:id;primaryKey;comment:ID"`
	// ...
}

func (*TestTableFieldCommentUpdate) TableName() string { return "test_table_field_comment" }