fix: None dataset and schema permissions

[dpgaspar]

SUMMARY

Currently load examples creates duplicate schema and dataset permissions like:

[None].[covid_vaccines](id:19)
[examples].[covid_vaccines](id:19)
[None].[channel_members](id:22)
[examples].[channel_members](id:22)
...

This is due to the fact that a SqlaTable can be created without an associated database triggering SQLAlchemy listen set_perm with a None database.
This is a simple/patch fix, we should revisit the entire logic around creating permissions for datasets, databases and schemas since it's possible to have orphaned permissions also when (for example) a database name is changed.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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

[codecov]

Codecov Report

Merging #20108 (7bab7f5) into master (660af40) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #20108      +/-   ##
==========================================
- Coverage   66.47%   66.46%   -0.01%     
==========================================
  Files        1721     1721              
  Lines       64477    64485       +8     
  Branches     6795     6795              
==========================================
+ Hits        42858    42862       +4     
- Misses      19891    19895       +4     
  Partials     1728     1728              

Flag	Coverage Δ
hive	53.70% <69.23%> (-0.01%)	⬇️
mysql	82.14% <100.00%> (+<0.01%)	⬆️
postgres	82.19% <100.00%> (-0.01%)	⬇️
presto	53.56% <69.23%> (-0.01%)	⬇️
python	82.61% <100.00%> (-0.01%)	⬇️
sqlite	81.94% <100.00%> (+<0.01%)	⬆️
unit	49.42% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	89.59% <100.00%> (+0.01%)	⬆️
superset/exceptions.py	90.90% <100.00%> (+0.08%)	⬆️
superset/security/manager.py	95.33% <100.00%> (+0.04%)	⬆️
superset/reports/commands/log_prune.py	85.71% <0.00%> (-3.58%)	⬇️
superset/commands/importers/v1/utils.py	92.20% <0.00%> (-1.30%)	⬇️
superset/common/query_object.py	94.73% <0.00%> (-0.53%)	⬇️
superset/reports/commands/execute.py	91.24% <0.00%> (-0.37%)	⬇️

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[villebro]

LGTM, this is a really welcome improvement!