Describe the bug
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version 1.16.4
when the commit history is long, I keep getting this issue, and finally the changelog can't generate correctly

/Users/xxx/.gem/gems/github_changelog_generator-1.16.4/lib/github_changelog_generator/octo_fetcher.rb:343:in `commits_in_branch': undefined method `map' for nil:NilClass (NoMethodError)

      @branches ||= client.branches(user_project).map { |branch| [branch[:name], branch] }.to_h
                                                 ^^^^

To Reproduce
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4. See error

Expected behavior
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Generate changelog successfully

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Desktop (please complete the following information):

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• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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