fs: flush option ignored when writing/appending strings

#50009 and #49884 conflict and were merged independently

This was exposed in https://github.com/nodejs/node/pull/60539 CI run

https://github.com/nodejs/node/blob/dbe45b767b0bb9c984206edd4ad0b5658d7c2fe4/test/parallel/test-fs-write-file-flush.js#L26-L37 fails if encoding is specified:

diff --git a/test/parallel/test-fs-write-file-flush.js b/test/parallel/test-fs-write-file-flush.js
index 98a8d637c5f..c28e536c528 100644
--- a/test/parallel/test-fs-write-file-flush.js
+++ b/test/parallel/test-fs-write-file-flush.js
@@ -26,7 +26,7 @@ test('synchronous version', async (t) => {
   await t.test('performs flush', (t) => {
     const spy = t.mock.method(fs, 'fsyncSync');
     const file = nextFile();
-    fs.writeFileSync(file, data, { flush: true });
+    fs.writeFileSync(file, data, { flush: true, encoding: 'utf8' });
     const calls = spy.mock.calls;
     assert.strictEqual(calls.length, 1);
     assert.strictEqual(calls[0].result, undefined);

Here, if options are set to e.g. { flush: true } (or any other object), they do not default to those values

https://github.com/nodejs/node/blob/dbe45b767b0bb9c984206edd4ad0b5658d7c2fe4/lib/fs.js#L2322-L2327

As getOptions does not merge options with the default ones, unless it's just an encoding string: https://github.com/nodejs/node/blob/dbe45b767b0bb9c984206edd4ad0b5658d7c2fe4/lib/internal/fs/utils.js#L318-L339

And the options.encoding === 'utf8' codepath was not called when options were set but options.encoding was undefined, as in flush testcases, resulting in the slow path

The bug was still triggered if both encoding and flush were set though

#60539 fixes that accidental slow path when options object without encoding is passed But it exposes the flush bug even when encoding is not set

And fast path doesn't handle options.flush at all