test: increase coverage of Buffer.transcode

Adds test for transcoding an empty buffer.

PR-URL: nodejs#10437
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-icu-transcode.js

@@ -70,3 +70,8 @@ assert.deepStrictEqual(
       buffer.transcode(uint8array, 'latin1', 'utf16le'),
       Buffer.from('hä', 'utf16le'));
 }
+
+{
+  const dest = buffer.transcode(new Uint8Array(), 'utf8', 'latin1');
+  assert.strictEqual(dest.length, 0);
+}