charset: fix data race in the Collation.IsDefault (#35370)
charset: fix data race in the Collation.IsDefault (#35370)

close pingcap/tidb#35145
ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,kyle-wang/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,ngaut/tidb