Coerce empty input value to empty string

[sanderhahn]

Makes input value work with undefined values (the input will not display "undefined" as text).

Fixes #3569

[codecov-io]

Codecov Report

Merging #3570 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3570   +/-   ##
=======================================
  Coverage   50.25%   50.25%           
=======================================
  Files           1        1           
  Lines         197      197           
=======================================
  Hits           99       99           
  Misses         98       98

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