fix(plugin-chart-echarts): show zero value in tooltip

[villebro]

SUMMARY

Currently only truthy values are shown on the tooltip of the ECharts plugin, causing zeros to be removed.

The PR fixes the bug and adds unit tests for both extracting the series and formatting (logic of both have been updated). In addition the tests are refactored to the preferred single test format (the refactoring changes can best be seen with "Hide whitespace changes").

AFTER

Now zero is shown on the tooltip; see the series SUM(0):

BEFORE

Previously only truthy values were shown, i.e. excluding zero:

TESTING INSTRUCTIONS

1. Create a chart using the ECharts plugin
2. Create a series that contains a zero value
3. Check that the value show up on the tooltip

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

[stephenLYZ]

LGTM!

[codecov]

Codecov Report

Merging #21296 (749e137) into master (034ee1c) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #21296      +/-   ##
==========================================
- Coverage   66.43%   66.42%   -0.02%     
==========================================
  Files        1784     1784              
  Lines       68185    68185              
  Branches     7265     7264       -1     
==========================================
- Hits        45298    45291       -7     
- Misses      21018    21024       +6     
- Partials     1869     1870       +1     

Flag	Coverage Δ
javascript	52.47% <50.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...plugins/plugin-chart-echarts/src/utils/forecast.ts	93.22% <50.00%> (ø)
superset-frontend/src/featureFlags.ts	28.57% <0.00%> (-71.43%)	⬇️
...rontend/src/components/ErrorMessage/ErrorAlert.tsx	89.47% <0.00%> (-0.78%)	⬇️
superset-frontend/src/SqlLab/actions/sqlLab.js	61.05% <0.00%> (-0.33%)	⬇️
...c/views/CRUD/data/database/DatabaseModal/index.tsx	32.03% <0.00%> (-0.17%)	⬇️
...d/src/SqlLab/components/SaveDatasetModal/index.tsx	52.87% <0.00%> (ø)
...onents/ErrorMessage/ErrorMessageWithStackTrace.tsx	60.00% <0.00%> (ø)
...-frontend/src/components/FilterableTable/index.tsx	72.63% <0.00%> (+0.29%)	⬆️

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