zig: remove JSValue.isEmpty

[nektro]

progress towards zig JSValue stability and safety
making these direct comparisons to .zero makes it clearer to the reader what we're dealing with
and eases the transition to remove .zero later

[robobun]

❌ @nektro, your commit 905f3b6 has 4 failures in #5979:

test/js/web/fetch/fetch.tls.test.ts - 1 failing on 🍎 aarch64

test/js/web/fetch/fetch-preconnect.test.ts - 1 failing on 🍎 aarch64

test/js/node/test/parallel/zlib.test.js - 1 failing on 🍎 aarch64

test/js/node/test/parallel/child-process-exec-timeout-kill.test.js - 1 failing on 🍎 aarch64