feat: skip check when html tag does not exists

[yeonjuan]

fixes #153

[codecov]

Codecov Report

Merging #154 (b8ac4e6) into main (aba345c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files          44       44           
  Lines         919      921    +2     
  Branches      231      232    +1     
=======================================
+ Hits          898      900    +2     
  Misses         18       18           
  Partials        3        3           

Flag	Coverage Δ
unittest	97.71% <100.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
...ackages/eslint-plugin/lib/rules/require-doctype.js	100.00% <100.00%> (ø)