fix(emptyTheme): avoid theme object mutation

[layershifter]

Fixes a small regression from #1837 that was mentioned by @levithomason.

???

[DustyTheBot]

	Warnings
⚠️	There are no updates provided to CHANGELOG. Ensure there are no publicly visible changes introduced by this PR.

Generated by 🚫 dangerJS

[codecov]

Codecov Report

Merging #1846 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1846      +/-   ##
==========================================
- Coverage   69.64%   69.63%   -0.01%     
==========================================
  Files         874      875       +1     
  Lines        7612     7611       -1     
  Branches     2216     2216              
==========================================
- Hits         5301     5300       -1     
  Misses       2303     2303              
  Partials        8        8

Impacted Files	Coverage Δ
packages/react/src/lib/renderComponent.tsx	84.74% <100%> (ø)	⬆️
packages/react/src/lib/createEmptyTheme.ts	100% <100%> (ø)
packages/react/src/lib/mergeThemes.ts	100% <100%> (ø)	⬆️

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[lucivpav]

How about testing this fix?

[jdhuntington]

Closing as this has seen no activity for awhile. Please reopen if still relevant. Thanks!