fix(legacy-plugin-chart-pivot-table): pivot table chart string aggregation empty values

[mayurnewase]

🐛 Bug Fix
Fixes; #868

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[codecov]

Codecov Report

Merging #880 (786e2df) into master (8b28452) will increase coverage by 0.14%.
The diff coverage is 53.12%.

@@            Coverage Diff             @@
##           master     #880      +/-   ##
==========================================
+ Coverage   26.58%   26.73%   +0.14%     
==========================================
  Files         380      405      +25     
  Lines        8192     8248      +56     
  Branches     1118     1126       +8     
==========================================
+ Hits         2178     2205      +27     
- Misses       5886     5914      +28     
- Partials      128      129       +1     

Impacted Files	Coverage Δ
.../legacy-plugin-chart-pivot-table/src/PivotTable.js	0.00% <0.00%> (ø)
...-plugin-chart-pivot-table/src/utils/formatCells.ts	70.83% <70.83%> (ø)
...plugin-chart-echarts/src/BoxPlot/transformProps.ts	50.00% <0.00%> (-2.64%)	⬇️
plugins/plugin-chart-table/test/testData.ts	83.33% <0.00%> (ø)
plugins/plugin-chart-table/src/controlPanel.tsx	37.03% <0.00%> (ø)
packages/superset-ui-core/src/models/Registry.ts	100.00% <0.00%> (ø)
plugins/plugin-chart-table/src/transformProps.ts	71.60% <0.00%> (ø)
...ackages/superset-ui-core/src/query/api/v1/index.ts	100.00% <0.00%> (ø)
...ckages/superset-ui-chart-controls/src/sections.tsx	100.00% <0.00%> (ø)
...gins/legacy-preset-chart-nvd3/src/NVD3Controls.tsx	0.00% <0.00%> (ø)
... and 60 more

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[villebro]

@mayurnewase this seems to reintroduce displaying of null values (see #839). It would be great to add a test that ensures that "null" cell values are replaced with "" and add a test to ensure that those are handled properly. Other than that looks great!

[villebro]

A few minor comments, other than that LGTM!

[mayurnewase]

@villebro Getting float values from db_engine's cursor.fetchall() for avg(any_string_column),it doesn't seem to be introduced by this pr.
in sql lab,

[villebro]

LGTM, thanks so much for fixing and improving this code @mayurnewase !

[ktmud]

Eh... I don't think this is a valid JavaScript syntax....