fix(plugin-chart-pivot-table): fix displaying column names when labels are available

[kgabryje]

Fixes apache/superset#14716

In order to test this PR, link plugin-chart-pivot-table to Superset and create a Pivot Table chart with rows and columns which have labels assigned in dataset

Before:

After:

CC: @junlincc

[vercel]

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[codecov]

Codecov Report

Merging #1132 (bd53c25) into master (e7219ec) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1132      +/-   ##
==========================================
- Coverage   28.99%   28.98%   -0.01%     
==========================================
  Files         462      462              
  Lines        9261     9262       +1     
  Branches     1472     1473       +1     
==========================================
  Hits         2685     2685              
- Misses       6366     6367       +1     
  Partials      210      210              

Impacted Files	Coverage Δ
...s/plugin-chart-pivot-table/src/PivotTableChart.tsx	0.00% <ø> (ø)
...gin-chart-pivot-table/src/plugin/transformProps.ts	71.42% <0.00%> (-11.91%)	⬇️

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[rusackas]

LGTM!