[BUG] choose broadcastjoin first when cardinality cannot be estimated

[wutiangan]

sql：
select * from (select k1 from jointest group by k1)t2, jointest t1 where t1.k1 = t2.k1

reason:
In version 0.11, the SQL choose broadcastjoin and returns it in 9s. However, in version 0.12, the SQL choose shuffle join and runs 32S.
When the cardinality cannot be estimated, it is better to choose broadcast join first. When there is a performance bottleneck, users can manually add hint to shuffle point.