ESLint: Re-enable rule no-non-null-assertion

[kgabryje]

SUMMARY

Re-enable ESLint rule no-non-null-assertion, which was disabled in PR #10839. Code was refactored to fix the errors raised by the rule.

TEST PLAN

Run npm run lint, verify that there are no new Javascript/Typescript errors.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

Merging #10864 into master will decrease coverage by 2.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #10864      +/-   ##
==========================================
- Coverage   61.33%   59.22%   -2.11%     
==========================================
  Files         380      776     +396     
  Lines       24068    37054   +12986     
  Branches        0     3309    +3309     
==========================================
+ Hits        14762    21947    +7185     
- Misses       9306    14922    +5616     
- Partials        0      185     +185     

Flag	Coverage Δ
#cypress	55.35% <0.00%> (?)
#python	61.32% <ø> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...src/components/FilterableTable/FilterableTable.tsx	2.64% <0.00%> (ø)
superset/db_engine_specs/sqlite.py	65.62% <0.00%> (-9.38%)	⬇️
superset/utils/celery.py	82.14% <0.00%> (-3.58%)	⬇️
superset/result_set.py	96.69% <0.00%> (-1.66%)	⬇️
superset-frontend/src/components/FormLabel.tsx	100.00% <0.00%> (ø)
superset-frontend/src/views/App.tsx	100.00% <0.00%> (ø)
.../explore/components/controls/DateFilterControl.jsx	59.88% <0.00%> (ø)
.../src/dashboard/components/menu/PopoverDropdown.jsx	25.00% <0.00%> (ø)
superset-frontend/src/views/menu.tsx	100.00% <0.00%> (ø)
...-frontend/src/visualizations/presets/MainPreset.js	100.00% <0.00%> (ø)
... and 391 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8a774d5...4bc3711. Read the comment docs.