prevent erroneous missing data warnings for custom elements

[Rich-Harris]

fixes #1065

[codecov-io]

Codecov Report

Merging #1154 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
+ Coverage   91.48%   91.49%   +<.01%     
==========================================
  Files         126      126              
  Lines        4524     4526       +2     
  Branches     1461     1462       +1     
==========================================
+ Hits         4139     4141       +2     
  Misses        163      163              
  Partials      222      222

Impacted Files	Coverage Δ
src/generators/dom/index.ts	95.53% <100%> (+0.05%)	⬆️

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