BUG: value_counts can handle the case even with empty groups (#28479)

[dongho-jung]

• If applying rep to recons_labels go fail, use ids which has no
  consecutive duplicates instead.

• closes value_counts() crashes if groupby object contains empty groups #28479
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

xuancong84 found that value_counts() crashes if groupby object contains empty groups.
However, even though I made the construction of DataFrame don't skip empty rows, it still crashed.
Till then, I already tried in many ways though, in this time I tried to correct the callee self.grouper.recons_labels. After several tests, I found that If freq of Grouper is too long so that it has empty groups in some periods then it crashes. And also have found that this is solved by using ids which has no consecutive duplicates instead of self.grouper.recons_labels.

[dongho-jung]

Sorry for the second commit... Can it be squashed..?

[jreback]

@0xF4D3C0D3 FYI you don't need to create a new PR, you can simply rebase and push to the existing one (leave this as its already done)

[dongho-jung]

@jreback Thanks for your advice, I just worried that if I had committed something like a trivial typo, it would have made the history of main repo dirty. So for the sake of only one commit, I checkout with another branch and commit with a random message and git reset --soft HEAD~2 and git commit --edit -m"$(git log --format=%B --reverse HEAD..HEAD@{1})" for squashing. You mean that I don't need all this fuss and it's ok that PR with multiple commits as it can be squashed when it merged?

[jreback]

You mean that I don't need all this fuss and it's ok that PR with multiple commits as it can be squashed when it merged?

yes you can do whatever you want, including merges & rebases on a branch; on merge we will squash to a single commit.

[dongho-jung]

Wow, that's cool! sorry for my inexperience D:
I'll practice and learn more to better contribution. Thanks

[jreback]

are there any perf implications here? e.g. show the run of the groupby asvs

[jreback]

lgtm. pls merge master and ping on green.

[jreback]

thanks @0xF4D3C0D3