Skip enqueue of empty chunks in TextDecoderStream

After BOM stripping or when receiving an empty Uint8Array, the
decoded input can be zero-length. Per spec, empty chunks should
produce no output rather than enqueuing an empty string.

src/browser/tests/streams/text_decoder_stream.html

@@ -59,3 +59,24 @@
     testing.expectEqual(true, result2.done);
   })();
 </script>
+
+<script id=text_decoder_stream_empty_chunk>
+  (async function() {
+    const tds = new TextDecoderStream();
+    const writer = tds.writable.getWriter();
+    const reader = tds.readable.getReader();
+
+    // Write an empty chunk followed by real data
+    await writer.write(new Uint8Array([]));
+    await writer.write(new Uint8Array([104, 105]));
+    await writer.close();
+
+    // Empty chunk should be filtered out; first read gets "hi"
+    const result = await reader.read();
+    testing.expectEqual(false, result.done);
+    testing.expectEqual('hi', result.value);
+
+    const result2 = await reader.read();
+    testing.expectEqual(true, result2.done);
+  })();
+</script>

src/browser/webapi/encoding/TextDecoderStream.zig

@@ -65,6 +65,9 @@ fn decodeTransform(controller: *TransformStream.DefaultController, chunk: js.Val
         input = input[3..];
     }
 
+    // Per spec, empty chunks produce no output
+    if (input.len == 0) return;
+
     try controller.enqueue(.{ .string = input });
 }