fix: allow document selector to be cleared

[Samtaro]

Previously, it did not allow the document field(tree select) to be cleared even if the field was not required.

[codecov]

Codecov Report

Merging #274 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #274   +/-   ##
=======================================
  Coverage   78.45%   78.45%           
=======================================
  Files         108      108           
  Lines        2177     2177           
  Branches      152      152           
=======================================
  Hits         1708     1708           
  Misses        451      451           
  Partials       18       18

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[birkir]

Thank you!