fs.mkdir empty string bugfix

[dylan-conway]

What does this PR do?

Fixes handling of empty strings in mkdir when recursive is true.

How did you verify your code works?

added tests

[robobun]

❌ @dylan-conway, your commit 4eb9da7 has 8 failures in #4890:

test/integration/next-pages/test/next-build.test.ts - 1 failing on 🐧 22.04 x64

test/js/bun/http/bun-serve-static.test.ts - 1 failing on 🍎 13 aarch64

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

test/js/node/net/node-net-server.test.ts - 1 failing on 🪟 x64

test/bundler/bundler_compile.test.ts - 2 failing on 🪟 x64

test/bundler/bundler_compile.test.ts - 3 failing on 🪟 x64-baseline

test/bundler/bundler_edgecase.test.ts - 1 failing on 🪟 x64

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

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