Fix flaky test_dataframe_aggregations_multilevel

[rjzamora]

I'm struggling to reproduce the recent groupby CI failures locally. However, it seems possible that these errors could be the result of _mul_cols being passed an empty group within apply. This PR adds a possible fix if this happens to be the root cause.

[rjzamora]

Update: Seems like this may fix the problem - Just added a comment, and re-running CI.

[jrbourbeau]

Woo, thanks @rjzamora! Just to clarify, this should close #8795, correct?

[rjzamora]

Just to clarify, this should close #8795, correct?

I think so, but not completely sure. Honestly not sure what else may be causing the problem.

[jrbourbeau]

I see a similar thing happening here

dask/dask/dataframe/groupby.py

Lines 640 to 646 in 49b516b

	def _drop_duplicates_reindex(df):
	# Fix index in a groupby().apply() context
	# https://github.com/dask/dask/issues/8137
	# https://github.com/pandas-dev/pandas/issues/43568
	result = df.drop_duplicates()
	result.index = [0] * len(result)
	return result

I'll suggest we update there too and then merge this PR in.

The change here seems consistent with fixing #8795 and also straightforward. We can always reopen #8795 if we see the flaky test failure again.

[jrbourbeau]

Thanks @rjzamora!