Add test for empty-trade-history edge case in the Benford engine

[Inkman007]

Description:
detection/benford_engine.py computes chi-square, per-digit Z-scores, and MAD scores from a wallet's trade amounts. It's not clear from the existing test suite (tests/test_benford.py) what happens when the input trade list is empty (a brand-new wallet with zero trades in the current rolling window). This should return a well-defined neutral result rather than raising an unhandled exception or producing NaNs that silently propagate into the feature matrix.

Acceptance Criteria:

• A new test in tests/test_benford.py calls the chi-square/Z-score/MAD functions with an empty amounts list.
• The test asserts either a documented sentinel value (e.g. 0.0 or None) is returned, or a specific, named exception is raised.
• If the current behavior is undesirable (raises a generic exception, returns NaN), the function is updated to handle the empty case explicitly and the test locks in the new behavior.
• Test passes under make test.

Relevant files:
detection/benford_engine.py, tests/test_benford.py

[shobhamerabacha-star]

Hello @maintainers,

I would love to take this on. My planned approach:

• Implement Add test for empty-trade-history edge case in the Benford engine with explicit require_auth checks, clean storage layouts, and robust Soroban contract logic.
• Add comprehensive unit tests verifying both happy-path invocations and unauthorized revert scenarios.

I am ready to start immediately—please assign this to me!