Add a missing instruction, and a testcase

test/nbt_test.dart

@@ -86,4 +86,13 @@ void main() {
     // Expect that the file exists
     expect(file.existsSync(), true);
   });
+
+  test("Write NULL UUID to NBT", () async {
+    CompoundTag tag = CompoundTag();
+    NbtUtils.writeUUID(tag, "test", NbtUUID.fromUUID(UUID.ZERO));
+    NbtUUID ID = NbtUtils.readUUID(tag, "test");
+
+    expect(ID.MSB, 0);
+    expect(ID.LSB, 0);
+  });
 }