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

  	ifcommit==nil {
  	returnnil, nil, errors.New("commit must be provided")
  	}

• go-github/github/pulls.go

  Lines 359 to 361 in 8dacfa5

  	ifpull==nil {
  	returnnil, nil, errors.New("pull must be provided")
  	}

• go-github/github/enterprise_manage_ghes_config.go

  Lines 466 to 468 in 6540c41

  	ifopts==nil {
  	returnnil, 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 

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[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