fix: dashboard datasources filter None

[etr2460]

SUMMARY

Mypy types led me astray and proved that dashboard.datasources was actually a Set[Optional[BaseDatasource]]. I've updated logic to filter out None datasources for dashboards, updated the types to be correct, and handle slice.datasource == None throughout the app

TEST PLAN

CI

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

to: @villebro @john-bodley @graceguo-supercat @ktmud @suddjian

[codecov]

Codecov Report

Merging #14471 (7007b57) into master (d138b44) will decrease coverage by 0.14%.
The diff coverage is 63.63%.

❗ Current head 7007b57 differs from pull request most recent head 4ab992c. Consider uploading reports for the commit 4ab992c to get more accurate results

@@            Coverage Diff             @@
##           master   #14471      +/-   ##
==========================================
- Coverage   77.11%   76.97%   -0.15%     
==========================================
  Files         952      952              
  Lines       48156    48161       +5     
  Branches     6054     6054              
==========================================
- Hits        37137    37071      -66     
- Misses      10818    10889      +71     
  Partials      201      201              

Flag	Coverage Δ
hive	?
mysql	81.05% <63.63%> (-0.01%)	⬇️
postgres	81.07% <63.63%> (-0.03%)	⬇️
presto	80.79% <63.63%> (-0.01%)	⬇️
python	81.36% <63.63%> (-0.28%)	⬇️
sqlite	80.69% <63.63%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
superset/views/core.py	75.49% <25.00%> (-0.15%)	⬇️
superset/views/utils.py	84.06% <50.00%> (ø)
superset/models/dashboard.py	77.98% <100.00%> (ø)
superset/models/slice.py	86.03% <100.00%> (+0.07%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	70.32% <0.00%> (-17.08%)	⬇️
superset/connectors/sqla/models.py	89.83% <0.00%> (-0.25%)	⬇️
superset/utils/core.py	88.77% <0.00%> (-0.13%)	⬇️

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