[405] Eliminate data skew in hash shuffle

[lgbo-ustc]

Changelog category (leave one):

• Bug Fix

Since cityHash64 will be null if any argument is null, this could cause data skew. Remove the codes for marking null rows to belong to partition 0, we still keep the result correct.

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

...

Documentation entry for user-facing changes

• [ ] Documentation is written (mandatory for new features)

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close #405

[kyligence-git]

Can one of the admins verify this patch?

[lgbo-ustc]

test this please

[lgbo-ustc]

test this please