Solve the programming task below in a Python markdown code block.
Given a non-negative number, return the next bigger polydivisible number, or an empty value like `null` or `Nothing`.

A number is polydivisible if its first digit is cleanly divisible by `1`, its first two digits by `2`, its first three by `3`, and so on. There are finitely many polydivisible numbers.
Also feel free to reuse/extend the following starter code:
```python
def next_num(n):
```