reafactor: add nil detection when sqldb return

[KantaHasegawa]

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

What did this pull request do?

This pull request addresses a bug that caused a panic when connecting to the database using an incorrect DSN. The bug has already been fixed in this commit (c1ea730). However, to prevent potential future bugs, I have refactored the code to include null detection during type assertion.

User Case Description

gorm/gorm.go

Lines 374 to 387 in c1ea730

	// DB returns `*sql.DB`
	func (db *DB) DB() (*sql.DB, error) {
	connPool := db.ConnPool
	if dbConnector, ok := connPool.(GetDBConnector); ok && dbConnector != nil {
	return dbConnector.GetDBConn()
	}
	if sqldb, ok := connPool.(*sql.DB); ok {
	return sqldb, nil
	}
	return nil, ErrInvalidDB
	}

Before merging this PR (c1ea730), connPool.(*sql.DB) returned nil, nil which had a negative impact on the called method.

[a631807682]

c1ea730 is ok to me, and this change is also the same, but in order to achieve the purpose of your appeal, we also need to detect the content returned by dbConnector.GetDBConn() to ensure that if *sql.DB is nil, an error must be returned , and you can revert the error judged code in c1ea730

[a631807682]

thanks for your contribution