fix: detect synchronous throws of falsy values in t.throwsAsync()

[tsushanth]

Bug

When the function passed to t.throwsAsync() throws a falsy value synchronously — for example throw 0, throw false, throw null, or throw "" — the guard that detects the synchronous throw uses a bare truthiness check:

let actual = null;
try {
    retval = thrower();
} catch (error) {
    actual = error;
}

if (actual) { // ← misses falsy thrown values

Because 0, false, null, and "" are all falsy, the if (actual) branch is skipped. The code falls through to the isPromise(retval) check (retval is undefined since the function threw), which also fails, so AVA ultimately reports "Function returned: undefined" instead of the correct "Function threw synchronously. Use t.throws() instead: ".

Fix

Change the sentinel check to actual !== null, matching the initial value assigned before the try/catch.

if (actual !== null) {

This correctly detects any synchronous throw — including falsy thrown values — without affecting the normal case where no exception is thrown (actual remains null).

[tsushanth]

The Node.js (^26, ubuntu-latest) failure appears pre-existing and unrelated to this PR — the main branch CI also has intermittent Node 26 failures (run #25970521309). This PR passes on Node 22, 24, and Node 26 on Windows and macOS.