fix(sqllab): Replace stringified 'null' schema column values with NULL

[john-bodley]

SUMMARY

I'm not sure exactly when this came about, though I believe the issue has been resolved recently (albeit not being able to identify the relevant PR(s)), but it seems at some stage we were stringifying an undefined (NULL) schema in SQL Lab as "null" in both the query and saved_query tables resulting in some undesirable user behavior.

This PR includes a migration which simply replaces the null string with NULL, i.e., undefined.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI and ran the upgrade/downgrade manually. Also ran the bench-marking script (thanks @betodealmeida) which produced the following results:

Migration for 1000+ entities took: 0.42 seconds

Results:

Current: 0.50 s
10+: 0.35 s
100+: 0.35 s
1000+: 0.42 s

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• 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

[john-bodley]

It's not evident why these tables use different string sizes for encoding the schema.

[etr2460]

yikes

[etr2460]

can you run Beto's db migration benchmarking script and post the results?

[etr2460]

yikes

[codecov]

Codecov Report

Merging #18992 (40c2332) into master (150fd0d) will decrease coverage by 0.19%.
The diff coverage is n/a.

❗ Current head 40c2332 differs from pull request most recent head 88af195. Consider uploading reports for the commit 88af195 to get more accurate results

@@            Coverage Diff             @@
##           master   #18992      +/-   ##
==========================================
- Coverage   66.58%   66.39%   -0.20%     
==========================================
  Files        1641     1641              
  Lines       63524    63524              
  Branches     6421     6421              
==========================================
- Hits        42299    42178     -121     
- Misses      19548    19669     +121     
  Partials     1677     1677              

Flag	Coverage Δ
hive	?
mysql	81.83% <ø> (ø)
postgres	81.88% <ø> (ø)
presto	?
python	81.93% <ø> (-0.39%)	⬇️
sqlite	?

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

Impacted Files	Coverage Δ
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	70.00% <0.00%> (-15.77%)	⬇️
superset/db_engine_specs/presto.py	83.47% <0.00%> (-5.65%)	⬇️
superset/db_engine_specs/sqlite.py	91.89% <0.00%> (-5.41%)	⬇️
superset/connectors/sqla/utils.py	88.23% <0.00%> (-3.93%)	⬇️
superset/utils/celery.py	86.20% <0.00%> (-3.45%)	⬇️
superset/result_set.py	96.77% <0.00%> (-1.62%)	⬇️
superset/databases/commands/test_connection.py	98.57% <0.00%> (-1.43%)	⬇️
superset/connectors/sqla/models.py	88.81% <0.00%> (-1.24%)	⬇️
superset/utils/cache.py	73.58% <0.00%> (-0.95%)	⬇️
... and 4 more

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[john-bodley]

@etr2460 I've addressed your comment.

[etr2460]

lgtm, thanks for the cleanup migration