Fix empty-pattern handling in KMP search

[Miladkhoshdel]

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?

Handle empty patterns in knuth_morris_pratt() by returning 0 before preprocessing, consistent with Python's str.find().

Previously, get_failure_array("") returned [0] without raising an exception, but this did not prevent the search from accessing pattern[0]. Consequently:

• knuth_morris_pratt("abc", "") raised IndexError.
• knuth_morris_pratt("", "") returned -1.

The early return fixes both cases. Existing doctests remain unchanged.

Validation:

• Existing doctests: 2 passed, 0 failed.
• 3,937 combinations of text and pattern matched str.find(), including empty strings.
• Ruff lint and formatting checks passed.

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.
• All new Python files are placed inside an existing directory. — Not applicable; no new files.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing. — Existing search doctests pass locally; the unchanged get_failure_array() helper has no doctest.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. — Not applicable; no new algorithm.
• 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". — Not applicable; no linked issue.