ERR: Fix segfault with .astype('category') on empty DataFrame

[jschendel]

• closes ERR: Segfault with df.astype(category) on empty dataframe #18004
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Credit to @cgohlke for determining the fix.

[jreback]

lgtm. ping on green.

if you are looking for things to work on #12860 is interesting.

[codecov]

Codecov Report

Merging #18015 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18015      +/-   ##
==========================================
- Coverage   91.23%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       50091    50091              
==========================================
- Hits        45703    45695       -8     
- Misses       4388     4396       +8

Flag	Coverage Δ
#multiple	89.03% <ø> (ø)	⬆️
#single	40.24% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.75% <0%> (-0.1%)	⬇️
pandas/io/pytables.py	92.84% <0%> (+0.04%)	⬆️

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[jschendel]

ping @jreback : green

Will look into the issue you referenced, seems interesting.

[jreback]

thanks @jschendel