refactor: Feature flags getter

[zhaoyongjie]

SUMMARY

The Feature flags getter has should be returned an explicit boolean value. The unit test makes a minor refactor.

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 #22015 (b7eb30f) into master (429f246) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #22015   +/-   ##
=======================================
  Coverage   65.76%   65.77%           
=======================================
  Files        1813     1813           
  Lines       69437    69423   -14     
  Branches     7450     7448    -2     
=======================================
- Hits        45667    45662    -5     
+ Misses      21850    21841    -9     
  Partials     1920     1920           

Flag	Coverage Δ
javascript	53.50% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...ackages/superset-ui-core/src/utils/featureFlags.ts	100.00% <100.00%> (ø)
superset-frontend/src/utils/downloadAsImage.ts	11.11% <0.00%> (-4.68%)	⬇️
...perset-frontend/src/addSlice/AddSliceContainer.tsx	59.61% <0.00%> (ø)
...d/src/filters/components/Time/TimeFilterPlugin.tsx	0.00% <0.00%> (ø)
.../legacy-plugin-chart-histogram/src/controlPanel.ts	40.00% <0.00%> (ø)
...ore/components/controls/DateFilterControl/types.ts	100.00% <0.00%> (ø)
...ents/controls/DateFilterControl/utils/constants.ts	100.00% <0.00%> (ø)
...nts/controls/DateFilterControl/DateFilterLabel.tsx	58.13% <0.00%> (+1.95%)	⬆️
superset-frontend/src/utils/common.js	100.00% <0.00%> (+10.00%)	⬆️

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[villebro]

Some code style opinions, LMKWYT

[villebro]

LGTM