feat(checkbox): the initial value of prefix

[zonemeen]

If the prefix property in the checkbox type is not set to any value, it will show undefined, which looks weird, so i think need to set a default value.

[codecov]

Codecov Report

Merging #1142 (a7a8bd0) into master (945e20a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1142   +/-   ##
=======================================
  Coverage   69.38%   69.38%           
=======================================
  Files           8        8           
  Lines         147      147           
  Branches       25       25           
=======================================
  Hits          102      102           
  Misses         27       27           
  Partials       18       18           

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[SBoudrias]

Thanks!