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 136 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.

[gmlewis]

Thank you, @tomfeigin !
Merging.

[felixlut]

Could we get a new release with this change as well? I have some changes over the last couple of weeks that will unblock some stuff I'm working on in https://github.com/integrations/terraform-provider-github:

• feat: support team organization role assignment integrations/terraform-provider-github#2322
• feat: support repository level custom_property resource and custom_properties datasource integrations/terraform-provider-github#2316

[gmlewis]

Could we get a new release with this change as well?

Yes, I can work on a new release later today.

[gmlewis]

Could we get a new release with this change as well?

Yes, I can work on a new release later today.

This has now been released here:
https://github.com/google/go-github/releases/tag/v66.0.0