Fixes [npm] Sorbet related issues in file fetcher

[sachin-sandhu]

What are you trying to accomplish?

Fixes following Sorbet errors

Return value: Expected type [String, String], got type [String, NilClass]
Caller: /home/dependabot/npm_and_yarn/lib/dependabot/npm_and_yarn/file_fetcher.rb:456
Definition: /home/dependabot/npm_and_yarn/lib/dependabot/npm_and_yarn/file_fetcher.rb:488 (Dependabot::NpmAndYarn::FileFetcher#convert_dependency_path_to_name)

Anything you want to highlight for special attention from reviewers?

How will you know you've accomplished your goal?

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[randhircs]

LGTM