fix: reject empty negative binary input - #15689

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

Bug fixed: bin_to_decimal("-") and bin_to_decimal(" - ") incorrectly returned 0 instead of raising ValueError.

Added validation to reject empty input after removing the optional negative sign, along with two regression doctests.

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