Fix ValueError for Empty DataFrames: Ensure Process Count is at Least 1

[Mithil467]

Since nb_item comes out as 0 for empty dataframes and series, we were returning an empty list from the chunk function. Hence, we were yielding nothing from our DataType.get_chunks method which caused our chunks list being empty and nb_workers = len(chunks) = 0.

Let me know if this fix seems good enough, and also if we need to add any new tests.

Fixes #115, fixes #141.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (4666931) 86.72% compared to head (7c98541) 91.28%.

Files	Patch %	Lines
pandarallel/progress_bars.py	50.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #245 +/- ## ========================================== + Coverage 86.72% 91.28% +4.55% ========================================== Files 12 12 Lines 580 585 +5 ========================================== + Hits 503 534 +31 + Misses 77 51 -26 ```

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[till-m]

Hi @Mithil467,

thanks for the PR. Could you please add a test for this case?

[Mithil467]

Sure. I noticed that when progress_bar is True, we get ZeroDivisionError. In order to fix that, I would need to know what we expect UI wise. I personally like [2] more than [1] as it gives a sense of success, but would like to know your opinions or if you want to do things differently. Hence, I have added [2] for now, let me know if it needs changes.

Consoles:

1. 
2. 

Notebook:

1. 
2. 

[till-m]

Personally, I prefer option 1.: The grey bar is neither failure (red) nor success (green), similarly, the processing of the empty DataFrame didn't really succeed or fail. @nalepae do you have an opinion on the matter?

[Mithil467]

@till-m @nalepae If you say so, I can make the necessary changes to implement option [1]. Should I go ahead?

[SiRumCz]

Hi dev team, any updates on this one?

[till-m]

My apologies, I am not maintaining this project anymore, hence me not responding. But I can make an exception and see this PR through.

I would suggest going with [1]. @Mithil467 could you kindly ping me when/if you've implemented that?

[Mithil467]

@till-m @nalepae I have implemented the progress bar change to type [1]. Please review.

[nalepae]

Pandaral·lel is looking for a maintainer! If you are interested, please open an GitHub issue.

[till-m]

Thanks for the contribution :)

[Mithil467]

Thanks for the help! @till-m