fix(pivot): default missing series to NULL_STRING

[villebro]

SUMMARY

BEFORE

On the chart data endpoint, missing series names are currently removed from the dataset:

AFTER

Now missing series names are replaced with <NULL> and the series show up on the chart.

TESTING INSTRUCTIONS

CI + new tests

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #14748 (0871ade) into master (29828f8) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #14748      +/-   ##
==========================================
- Coverage   77.53%   77.45%   -0.08%     
==========================================
  Files         960      960              
  Lines       48744    48745       +1     
  Branches     6122     6122              
==========================================
- Hits        37793    37756      -37     
- Misses      10745    10783      +38     
  Partials      206      206              

Flag	Coverage Δ
hive	81.10% <100.00%> (+<0.01%)	⬆️
mysql	81.38% <100.00%> (+<0.01%)	⬆️
postgres	81.40% <100.00%> (+<0.01%)	⬆️
presto	?
python	81.78% <100.00%> (-0.15%)	⬇️
sqlite	81.02% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/utils/pandas_postprocessing.py	84.46% <100.00%> (+0.46%)	⬆️
superset/db_engine_specs/presto.py	84.42% <0.00%> (-5.48%)	⬇️
superset/connectors/sqla/models.py	88.70% <0.00%> (-1.45%)	⬇️
superset/models/core.py	89.70% <0.00%> (-0.27%)	⬇️

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[dpgaspar]

Nice!

[zhaoyongjie]

LGTM