fix Buffer.fill with a non-null empty fill including uninitialized bytes

[nektro]

pulled out of #13757 to merge independently

added new test

[robobun]

❌ @nektro, your commit 7d84ea3 has 11 failures in #4009:

test/js/node/watch/fs.watch.test.ts - timeout on 🍎 14 x64

test/js/web/streams/streams.test.js - 1 failing on 🍎 13 x64

test/js/node/test/parallel/worker-cleanexit-with-moduleload.test.js - 1 failing on 🐧 20.04 aarch64

test/js/web/fetch/fetch-tcp-stress.test.ts - timeout on 🍎 14 aarch64

test/js/web/fetch/fetch-tcp-stress.test.ts - timeout on 🍎 14 x64

test/js/node/test/parallel/fs-watch-recursive-add-file-with-url.test.js - 1 failing on 🍎 14 aarch64

test/js/web/workers/worker.test.ts - 1 failing on 🪟 x64-baseline

test/cli/hot/watch.test.ts - 1 failing on 🪟 x64-baseline

test/cli/hot/watch.test.ts - 1 failing on 🪟 x64

test/js/bun/http/fetch-file-upload.test.ts - 1 failing on 🪟 x64-baseline

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 13 aarch64

test/integration/next-pages/test/dev-server.test.ts - timeout on 🍎 14 aarch64

test/cli/watch/watch.test.ts - 2 failing on 🪟 x64-baseline

test/cli/watch/watch.test.ts - 2 failing on 🪟 x64

test/js/bun/http/bun-serve-static.test.ts - 5 failing on 🍎 13 aarch64