Fix .transform crash when SeriesGroupBy is empty (#26208)

[HHest]

• closes SeriesGroupBy.transform cannot handle empty series #26208
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #26228 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26228      +/-   ##
==========================================
- Coverage   91.97%   91.97%   -0.01%     
==========================================
  Files         175      175              
  Lines       52379    52381       +2     
==========================================
- Hits        48178    48176       -2     
- Misses       4201     4205       +4

Flag	Coverage Δ
#multiple	90.52% <100%> (ø)	⬆️
#single	40.7% <0%> (-0.15%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/generic.py	89.05% <100%> (+0.02%)	⬆️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 64104ec...d86d07b. Read the comment docs.

[codecov]

Codecov Report

Merging #26228 into master will decrease coverage by 0.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26228      +/-   ##
==========================================
- Coverage   92.04%   91.68%   -0.36%     
==========================================
  Files         175      174       -1     
  Lines       52289    50751    -1538     
==========================================
- Hits        48130    46532    -1598     
- Misses       4159     4219      +60

Flag	Coverage Δ
#multiple	90.19% <100%> (-0.4%)	⬇️
#single	41.16% <0%> (+0.3%)	⬆️

Impacted Files	Coverage Δ
pandas/core/groupby/generic.py	89.03% <100%> (+0.02%)	⬆️
pandas/core/panel.py	17.49% <0%> (-17.65%)	⬇️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/compat/numpy/function.py	90.39% <0%> (-0.41%)	⬇️
pandas/core/generic.py	93.34% <0%> (-0.24%)	⬇️
pandas/core/frame.py	97.02% <0%> (-0.12%)	⬇️
pandas/core/series.py	93.67% <0%> (ø)	⬆️
pandas/io/parsers.py
pandas/core/indexes/base.py	96.72% <0%> (ø)	⬆️
pandas/core/arrays/integer.py	96.35% <0%> (+0.02%)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 971dcc1...807b7cc. Read the comment docs.

[jreback]

can you check if we have an open issue for this?

[HHest]

can you check if we have an open issue for this?

Yes it is #26208

[pep8speaks]

Hello @HHest! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-05-14 17:10:18 UTC

[jreback]

@HHest lgtm. ping on green.

[HHest]

sd

@HHest lgtm. ping on green.

Thank you very much. Green.

[jreback]

thanks @HHest