Update longest_common_substring.py

[Alfredoxrock]

• Combined the ans_index and ans_length into a single tuple to track the best match (position + length) more cleanly.

• Early exit for empty strings.

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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• All function parameters and return values are annotated with Python type hints.
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• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[mindaugl]

This is a cosmetic change to the original implementation of the algorithm.