BUG: handle empty lists in json_normalize

[rgbkrk]

• closes json_normalize should be able to accept an empty list #15534
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

[rgbkrk]

I'll come back to this for the other checkboxes.

[jreback]

you'll want to rebase on master as travis was having some issues

[codecov-io]

Codecov Report

Merging #15535 into master will decrease coverage by -0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #15535      +/-   ##
==========================================
- Coverage   91.07%   91.04%   -0.03%     
==========================================
  Files         136      136              
  Lines       49165    49167       +2     
==========================================
- Hits        44775    44764      -11     
- Misses       4390     4403      +13

Impacted Files	Coverage Δ
pandas/io/json/normalize.py	97.91% <100%> (+0.04%)	✅
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.78% <0%> (-0.35%)	❌
pandas/util/testing.py	81.87% <0%> (-0.19%)	❌
pandas/core/frame.py	97.83% <0%> (-0.1%)	❌

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[rgbkrk]

Added a test and appended to the whatsnew. Test passes locally for me, time to wait on CI.

[jorisvandenbossche]

@rgbkrk Thanks!

[rgbkrk]

No problem, thank you!