Handle empty bulk enqueue

[caomengxuan666]

Summary

• make enqueue_bulk(value, 0) a no-op
• make enqueue_bulk(iterator, 0) and enqueue_bulk(begin, end) with an empty range no-ops
• add regression coverage for empty bulk enqueue inputs

Validation

• cmake -S . -B out/build/empty-bulk -G Ninja -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Release -DFETCHCONTENT_SOURCE_DIR_GOOGLEBENCHMARK=... -DFETCHCONTENT_SOURCE_DIR_GOOGLETEST=... -DFETCHCONTENT_SOURCE_DIR_HDRHISTOGRAM=...
• cmake --build out/build/empty-bulk --target mpsc_tests reaches link step after compiling the changed source and tests

Note: full link/test execution is blocked on Windows clang because current master links atomic.lib for non-MSVC compilers; this is unrelated to this change and is addressed separately in PR #2.