fix: reject a lone minus in hex_to_decimal - #15714

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?

hex_to_decimal("-") returned 0. After the leading minus is stripped, the remaining string is empty, and all([]) is True, so the function treated a lone minus as a valid value of zero. oct_to_decimal("-") already raises ValueError for the same input.

The new doctest only covers this rejected input. Existing examples are unchanged.

Checklist

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• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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