LibWeb: Make 'optional BufferSource' IDL arguments actually optional

Previously this was compiled to require an object despite the IDL file
specifying 'optional'.
This commit makes IDLGenerator respect this modifier, and fixes the only
affected instance.

Tests/LibWeb/Text/input/TextDecoder/TextDecoder_decode.html

@@ -0,0 +1,12 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        try {
+            let decoder = new TextDecoder("utf-8");
+            println(`[${decoder.decode(new Uint8Array([0x41, 0x42, 0x43]))}]`); // "ABC"
+            println(`[${decoder.decode()}]`);
+        } catch(e) {
+            println("ERROR: " + e.name + ": " + e.message);
+        }
+    });
+</script>