chore(datasets): Sanitizing /save response

[craig-rueda]

SUMMARY

Ensures a default dict will always be included in dataset saves. Needed to prevent a few edge cases.

[codecov]

Codecov Report

Merging #17579 (93fb828) into master (bfba4f1) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 93fb828 differs from pull request most recent head f023cc5. Consider uploading reports for the commit f023cc5 to get more accurate results

@@            Coverage Diff             @@
##           master   #17579      +/-   ##
==========================================
- Coverage   77.01%   76.99%   -0.02%     
==========================================
  Files        1049     1049              
  Lines       56683    56688       +5     
  Branches     7852     7852              
==========================================
- Hits        43652    43645       -7     
- Misses      12778    12790      +12     
  Partials      253      253              

Flag	Coverage Δ
hive	81.61% <100.00%> (+<0.01%)	⬆️
mysql	?
postgres	82.04% <100.00%> (+<0.01%)	⬆️
presto	81.90% <100.00%> (+<0.01%)	⬆️
python	82.49% <100.00%> (-0.04%)	⬇️
sqlite	81.72% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
superset/views/core.py	76.92% <100.00%> (-0.51%)	⬇️
superset/views/datasource/views.py	90.21% <100.00%> (+0.10%)	⬆️
superset/views/utils.py	82.26% <100.00%> (+0.45%)	⬆️
superset/db_engine_specs/mysql.py	94.04% <0.00%> (-3.58%)	⬇️
superset/models/core.py	89.26% <0.00%> (-0.74%)	⬇️

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[sadpandajoe]

🏷️ 2021.44