Fix empty input in parallel odd-even transposition sort

[sujaljangde22]

Describe your change

Passing an empty list to odd_even_transposition raises IndexError when the function accesses arr[0]. This change returns the empty list before creating worker processes.

Validation: reproduced the error before the fix. During development, all 11 doctests passed with an empty-input example included. The added example was subsequently removed to keep code and test changes separate as requested by this template.

Implemented with AI-assisted guidance; I reviewed and tested the change.

Replaces #15537 and #15514, which the bot closed for invalid checklist formatting. This submission uses the exact Markdown checkbox syntax.

• 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.
• All new Python files are placed inside an existing directory.
• 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.
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No new files or algorithms are introduced. Existing type annotations are unchanged; GitHub checks are pending. This bug fix does not close the separate sorting type-hint tracking issue.