BUG: Copy categorical codes if empty (fixes #18051)

[topper-123]

In agreement with @ghasemnaddaf , I've finished up his PR #18279 (added correct tests). Is it time still to get this into v0.21.1?

• [ x] closes BUG: method .nunique on categorical series in v0.21 with only NaNs gives ValueError #18051
• [ x] tests added / passed
• [x ] passes git diff upstream/master -u -- "*.py" | flake8 --diff
• [ x] whatsnew entry

[jreback]

lgtm. ping on green.

[topper-123]

All are green, @jreback .

[codecov]

Codecov Report

Merging #18436 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #18436      +/-   ##
==========================================
- Coverage   91.35%   91.34%   -0.02%     
==========================================
  Files         163      163              
  Lines       49681    49691      +10     
==========================================
+ Hits        45386    45388       +2     
- Misses       4295     4303       +8

Flag	Coverage Δ
#multiple	89.14% <100%> (ø)	⬆️
#single	39.67% <0%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/categorical.py	95.66% <100%> (ø)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.8% <0%> (-0.1%)	⬇️
pandas/io/formats/format.py	96.01% <0%> (ø)	⬆️
pandas/core/reshape/reshape.py	100% <0%> (ø)	⬆️
pandas/core/indexes/multi.py	96.41% <0%> (ø)	⬆️
pandas/core/indexes/datetimes.py	95.49% <0%> (+0.09%)	⬆️
pandas/core/dtypes/missing.py	90.74% <0%> (+0.11%)	⬆️

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[jreback]

thanks @topper-123 keep em coming!

[topper-123]

And thanks to @ghasemnaddaf for actually finding and correcting this bug!

[ghasemnaddaf]

urw :-) thx for taking over