Last Analyze Time is incorrectly set when using stats async load

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int);
insert into t values(1);
alter table t add column b int default 1;
set @@tidb_stats_load_sync_wait=0;
explain select * from t where a = 1 and b = 1;
show stats_meta;

2. What did you expect to see? (Required)

The last analyze timestamp of the table t should be NULL

3. What did you see instead (Required)

It has a non-NULL value.

4. What is your TiDB version? (Required)

master