fix(plugin-chart-echarts): fix opacity on area chart

[villebro]

🐛 Bug Fix

#1447 introduced a regression to area charts which broke both area and confidence bands. The reason for the original change was to remove area if it's not needed (a 0 opacity made it impossible to hover series underneath the series).

AFTER

BEFORE

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[codecov]

Codecov Report

Merging #1464 (ff5262f) into master (ba2703f) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1464   +/-   ##
=======================================
  Coverage   30.91%   30.91%           
=======================================
  Files         503      503           
  Lines       10215    10214    -1     
  Branches     1761     1760    -1     
=======================================
  Hits         3158     3158           
+ Misses       6805     6804    -1     
  Partials      252      252           

Impacted Files	Coverage Δ
...lugin-chart-echarts/src/Timeseries/transformers.ts	42.24% <0.00%> (+0.36%)	⬆️

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[kgabryje]

LGTM