fix: empty document should not panic on String()

[bcho]

fixes #629

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[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.36%. Comparing base (4f6cb1c) to head (b1de640).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #630   +/-   ##
=======================================
  Coverage   77.36%   77.36%           
=======================================
  Files          22       22           
  Lines        7885     7885           
=======================================
  Hits         6100     6100           
  Misses       1368     1368           
  Partials      417      417           

[goccy]

Thank you for your contribution ! LGTM 👍