FIX a11y setup being undefined

[ndelangen]

Issue: a11y config when not set can cause an error

What I did

Fix it by adding a default config of {}

[ndelangen]

@shilman

[codecov]

Codecov Report

Merging #5724 into next will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             next    #5724      +/-   ##
==========================================
- Coverage   33.28%   33.27%   -0.01%     
==========================================
  Files         648      648              
  Lines        9371     9372       +1     
  Branches     1330     1355      +25     
==========================================
  Hits         3119     3119              
- Misses       5637     5638       +1     
  Partials      615      615

Impacted Files	Coverage Δ
addons/a11y/src/index.js	0% <0%> (ø)	⬆️

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