[Feature Anywhere] Fix visibleVisLayers ser/deser

[ohltyler]

Description

The change to using a Map for persisting visible VisLayers in the vega spec config had a bug where during the string serialization/deserialization (necessary for passing through the expression functions), the Map obj could not be properly stringified which was leading to an empty Map when parsing in the VegaParser. This was causing no VisLayers to be able to render.

This fixes that by converting the Map => array when serializing, and converting back to a Map when deserializing in the parser.

Note another option could have been using a basic JS obj {} for persisting this, but that would limit us in not being able to use the VisLayerTypes enum as keys, and would need other conversion as well.

Also hardens a helper method by handling a case where events is undefined or empty, + a regression test for that.

Issues Resolved

[List any issues this PR will resolve]

Check List

• Commits are signed per the DCO using --signoff

[codecov-commenter]

Codecov Report

Merging #3758 (c15af26) into feature/feature-anywhere (8a2cc69) will increase coverage by 0.00%.
The diff coverage is 50.00%.

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@@                    Coverage Diff                    @@
##           feature/feature-anywhere    #3758   +/-   ##
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  Coverage                     66.50%   66.51%           
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  Files                          3225     3225           
  Lines                         61980    61982    +2     
  Branches                       9542     9543    +1     
=========================================================
+ Hits                          41221    41225    +4     
+ Misses                        18473    18471    -2     
  Partials                       2286     2286           

Flag	Coverage Δ
Linux	66.45% <50.00%> (+<0.01%)	⬆️
Windows	66.45% <50.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
...ins/vis_type_vega/public/data_model/vega_parser.ts	77.23% <0.00%> (-0.64%)	⬇️
src/plugins/vis_augmenter/public/test_constants.ts	100.00% <100.00%> (ø)
src/plugins/vis_augmenter/public/vega/helpers.ts	97.95% <100.00%> (-0.03%)	⬇️

... and 2 files with indirect coverage changes

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[ananzh]

Nice!