When alter column from String

When alter column from String to Nullable(String), we will not do mutation to the data. But for uniq aggregate function, it use different data strucuture: for nullable column, it will use AggregateFunctionNull with a nested uniq aggregator. the AggregateFunctionNull will serialize an extra bool flag. This will make the statistics file incompatible. The fix is add a flag to record if it is a nullable column during serialization. The format of statisticis has changed, and the server can fail if we have statistics with old format. This PR #90904 will fix the crash, and throw exception when the existing statisticis uses lagecy format. To avoid exception, we should run ALTER TABLE table MATERIALIZE STATISTICS ALL to regenerate the statistics to fix it. #90311 (Han Fei).