Fix C binding empty byte buffers

[fallintoplace]

Summary

• Handle empty C opendal_bytes without constructing a Rust slice from a null pointer.
• Return an OpenDAL error when non-empty input buffers have null data.
• Clarify C byte buffer ownership docs and add focused coverage for operator and writer writes.

Why

C callers can reasonably pass { .data = NULL, .len = 0, .capacity = 0 } for an empty buffer. Rust still requires slice::from_raw_parts to receive a non-null, aligned pointer even when the length is zero, so the old conversion could invoke undefined behavior.

Tests

• cargo clippy -- -D warnings
• cmake -S . -B build -DTEST_ENABLE_ASAN=ON -DFEATURES=opendal/services-memory && cmake --build build -j4 && ./build/tests
• cargo build --features="opendal/services-memory" && make -C tests clean all && OPENDAL_TEST=memory make -C tests behavior_test