fix: Add validation for nil inputs in various services

[alexandear]

Add nil checks to avoid nil pointer dereference and for consistency with other places:

• go-github/github/git_commits.go

  Lines 130 to 132 in 8dacfa5

  	if commit == nil {
  	return nil, nil, errors.New("commit must be provided")
  	}

• go-github/github/pulls.go

  Lines 359 to 361 in 8dacfa5

  	if pull == nil {
  	return nil, nil, errors.New("pull must be provided")
  	}

• go-github/github/enterprise_manage_ghes_config.go

  Lines 466 to 468 in 6540c41

  	if opts == nil {
  	return nil, errors.New("opts should not be nil")
  	}

[codecov]

Codecov Report

Attention: Patch coverage is 82.22222% with 24 lines in your changes missing coverage. Please review.

Project coverage is 91.26%. Comparing base (6540c41) to head (f56e6b5).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
github/codespaces_secrets.go	27.27%	16 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3636      +/-   ##
==========================================
- Coverage   91.33%   91.26%   -0.08%     
==========================================
  Files         184      184              
  Lines       16174    16309     +135     
==========================================
+ Hits        14773    14884     +111     
- Misses       1227     1243      +16     
- Partials      174      182       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @alexandear!
LGTM.

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

@stevehipwell - might you have time for a code review? Thank you!

[stevehipwell]

LGTM