fix: Allow removing/unsetting repository custom property

[felixlut]

This change will allow a user to set null when calling the create/update repository custom property endpoint. As written in the docs, doing so will remove/unset the custom property. Currently doing so is not possible

Normally I think this would count as a breaking change, since this will modify the behavior when omitting the Value, but I believe this is a straight up bug, as omitting the Value should always result in an error. So there shouldn't be any usages that's relying on this behavior. Technically it's possible, but improbable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.05%. Comparing base (2b8c7fa) to head (e294fd7).
Report is 135 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3309      +/-   ##
==========================================
- Coverage   97.72%   93.05%   -4.67%     
==========================================
  Files         153      172      +19     
  Lines       13390    11772    -1618     
==========================================
- Hits        13085    10955    -2130     
- Misses        215      723     +508     
- Partials       90       94       +4     

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[gmlewis]

Thank you, @felixlut !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.