Enable BranchRestriction fields to be empty

[michelangelomo]

Description

I've encountered an issue trying to update BranchRestriction on a personal repository. Github doesn't allow teams and users fields in the request payload.

How to reproduce

p := github.ProtectionRequest{
	Restrictions: &github.BranchRestrictionsRequest{
		Apps: []string{"myapp"},
	},
}
client.Repositories.UpdateBranchProtection(ctx, org, repo, branch, p)

will return

No subschema in "anyOf" matched.
For 'properties/users', nil is not an array.
For 'properties/teams', nil is not an array.
Not all subschemas of "allOf" matched.
For 'anyOf/1', {"users"=>nil, "teams"=>nil, "apps"=>["myapp"]} is not a null.

but if I set Teams and Users fields as []string{} Github returns 422 Validation Failed [{Resource: Field: Code: Message:Only organization repositories can have users and team restrictions}] as described above.

Fix

Allow Teams and Users fields to be empty by using omitempty.

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[codecov]

Codecov Report

Merging #1530 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1530   +/-   ##
=======================================
  Coverage   68.03%   68.03%           
=======================================
  Files          94       94           
  Lines        8528     8528           
=======================================
  Hits         5802     5802           
  Misses       1844     1844           
  Partials      882      882           

Impacted Files	Coverage Δ
github/repos.go	70.04% <ø> (ø)

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[michelangelomo]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[gmlewis]

Thank you, @michelangelomo !
LGTM.

Awaiting second LGTM before merging.

[wesleimp]

LGTM 👌

[gmlewis]

Thank you, @wesleimp !
Merging.