fix(explore): handle null control sections

[villebro]

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

The recent PR #20097 added explicit support for null-valued control panel sections. However, due to lacking TS coverage and type declarations in the codebase, these weren't always being checked properly. This caused Explore to fail to render when opening a Mixted timeseries chart with the GENERIC_CHART_AXES feature flag disabled.

This PR adds some missing types and filtering using the new type guard introduced in the aforementioned PR to ensure null sections are always skipped. A null value is also added to a control panel section fixture to make sure it doesn't break the utils.

Tip: enable "Hide whitespace" when reviewing the diff:

Note that this regression uncovered a pretty serious mistyping of the chart control panel config registry, but fixing it is beyond the scope of this PR.

TESTING INSTRUCTIONS

1. Disable the GENERIC_CHART_AXES feature flag
2. Open a mixed timeseries chart
3. Notice Explore fail to render

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

[kgabryje]

LGTM

[codecov]

Codecov Report

Merging #20142 (cd9d795) into master (0bcc21b) will not change coverage.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##           master   #20142   +/-   ##
=======================================
  Coverage   66.46%   66.46%           
=======================================
  Files        1721     1721           
  Lines       64512    64512           
  Branches     6806     6806           
=======================================
  Hits        42875    42875           
  Misses      19905    19905           
  Partials     1732     1732           

Flag	Coverage Δ
javascript	51.33% <90.00%> (ø)

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Impacted Files	Coverage Δ
...perset-frontend/src/utils/getControlsForVizType.js	92.85% <87.50%> (ø)
...ntend/src/explore/controlUtils/getControlConfig.ts	100.00% <100.00%> (ø)
superset-frontend/src/explore/fixtures.tsx	66.66% <100.00%> (ø)

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