Set ignore floating-point error in _get_join_keys().

[yeemey]

• closes merging two empty dataframes can incur a division by zero #17776
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #17838 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #17838      +/-   ##
==========================================
+ Coverage   91.22%   91.23%   +0.01%     
==========================================
  Files         163      163              
  Lines       49994    50016      +22     
==========================================
+ Hits        45606    45632      +26     
+ Misses       4388     4384       -4

Flag	Coverage Δ
#multiple	89.04% <100%> (+0.03%)	⬆️
#single	40.24% <44.44%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/core/sparse/series.py	95.26% <ø> (ø)	⬆️
pandas/core/series.py	94.98% <ø> (-0.01%)	⬇️
pandas/core/panel.py	96.94% <ø> (ø)	⬆️
pandas/core/frame.py	97.77% <100%> (-0.08%)	⬇️
pandas/core/reshape/merge.py	94.28% <100%> (+0.01%)	⬆️
pandas/core/generic.py	92.09% <100%> (ø)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/indexes/datetimes.py	95.58% <0%> (+0.09%)	⬆️
pandas/plotting/_converter.py	65.2% <0%> (+1.81%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #17838 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #17838      +/-   ##
==========================================
+ Coverage   91.22%   91.23%   +<.01%     
==========================================
  Files         163      163              
  Lines       49994    50016      +22     
==========================================
+ Hits        45606    45631      +25     
+ Misses       4388     4385       -3

Flag	Coverage Δ
#multiple	89.03% <100%> (+0.02%)	⬆️
#single	40.24% <44.44%> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/core/series.py	94.98% <ø> (-0.01%)	⬇️
pandas/core/panel.py	96.94% <ø> (ø)	⬆️
pandas/core/sparse/series.py	95.26% <ø> (ø)	⬆️
pandas/core/frame.py	97.77% <100%> (-0.08%)	⬇️
pandas/core/generic.py	92.09% <100%> (ø)	⬆️
pandas/core/reshape/merge.py	94.28% <100%> (+0.01%)	⬆️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	65.2% <0%> (+1.81%)	⬆️

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d12a7a0...6110d01. Read the comment docs.