POC: Remove unnecessary null check for `GroupColumn`

[Rachelint]

Which issue does this PR close?

Closes #. POC for checking #12944 , not for merged

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[Rachelint]

I found it actually now a trivial change. The challenge is that, we should refactor codes to support vectorize append firstly (otherwise, we need to add a branch for row's appending, and as a reuslt, we remove a row level branch and add another...)

I am trying it.

[Rachelint]

It is poc version of #12996