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")
  	}