fix(fetch): fallback to default UA and CA data for Deno.createHttpClient()

[AaronO]

Fixes #9827

Correct behaviour after fix

❯ cat <<EOF | ./target/debug/deno run -A --unstable -
const client = Deno.createHttpClient({});
const response = await fetch(
    "https://dump-headers.herokuapp.com/",
    { client },
);
const data = await response.json();
console.log(data["user-agent"]);
EOF
Deno/1.8.1

[AaronO]

@lucacasonato Added a regression test: c07c2d0

[lucacasonato]

LGTM. Thanks for the fix.