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fix(bigint): treat empty octets as zero in BigInt::from_octets (#4115)

An empty big-endian magnitude naturally denotes zero, and mainstream
bigint APIs agree: Java's BigInteger(signum, magnitude) permits a
zero-length magnitude for any signum, Python's int.from_bytes(b"")
returns 0, and Rust's num-bigint normalizes Sign::Minus with an empty
magnitude to zero. Previously MoonBit was the outlier, aborting with
"empty octet string" whenever signum != 0, while still accepting the
morally identical all-zero magnitude (e.g. b"\x00" with signum=-1).

Relax the panic on both backends to return zero, update the docs, and
replace the panic tests (bigint and builtin) with value tests covering
every signum.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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