API: empty map should not infer

[jreback]

closes #18509

[jreback]

@jorisvandenbossche @jschendel

[codecov]

Codecov Report

Merging #18517 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #18517      +/-   ##
==========================================
- Coverage   91.46%   91.45%   -0.02%     
==========================================
  Files         157      157              
  Lines       51447    51439       -8     
==========================================
- Hits        47055    47042      -13     
- Misses       4392     4397       +5

Flag	Coverage Δ
#multiple	89.32% <100%> (ø)	⬆️
#single	40.61% <0%> (-0.08%)	⬇️

Impacted Files	Coverage Δ
pandas/util/testing.py	82.18% <ø> (+0.28%)	⬆️
pandas/core/indexes/base.py	96.45% <100%> (-0.02%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️
pandas/core/base.py	96.77% <0%> (+0.23%)	⬆️

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[jorisvandenbossche]

Sorry for the late reply, but +1 !