Ensure map_partitions returns Series object if function returns scalar

[fjetter]

• Closes Map partitions on a series that yields a scalar per partition cannot be computed dask-expr#625
• Tests added / passed
• Passes pre-commit run --all-files

[phofl]

Lgtm

[fjetter]

Looks like there are a bunch of genuine typing issues since I set the dtype on the created series explicitly. @phofl if you want to, you can pick this up after the holidays but if not I'll handle it once I'm back

[phofl]

I'll take a look after the holidays, the dtype issue is most likely because of running this on an empty meta object, that's the situation where I've seen us ending up with NaN there

[fjetter]

@phofl in case you want to have another look

[fjetter]

I ended up removing the explicit dtype since it caused issues and was quite unnecessary considering that df is already the final result data. The casting/coercing logic of the Series takes care of compat stuff with nulls, etc.

[fjetter]

When using map_partitions internally we sometimes rely on the data not being cast to a Series. The cumulative aggregation logic becomes quite cumbersome otherwise. I hope this is not introducing any other weird artefacts 🤞

[phofl]

the logic looks fine, I think the test failures are something that we can fix by adjusting the test

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

     15 files  ± 0       15 suites  ±0   3h 35m 57s ⏱️ + 13m 1s
 12 934 tests + 1   11 938 ✅ +11     922 💤 ±0   73 ❌  - 10   1 🔥 ±0 
159 683 runs  +12  143 003 ✅ +20  16 508 💤 +2  132 ❌  - 10  40 🔥 ±0 

For more details on these failures and errors, see this check.

Results for commit c60269b. ± Comparison against base commit 1ecf464.

♻️ This comment has been updated with latest results.