Fix division by zero in NotebookProgressPar
# What does this PR do?

This PR fixes the bug reported in #12950. More precisely, the following snippet of code was failing with a division by zero error:
```py
from transformers.utils.notebook import NotebookProgressBar

pbar = NotebookProgressBar(total=1)
pbar.update(1)
pbar.update(1, force_update=True)
```

This PR fixes that by being a bit more defensive before dividing by a potential zero.

Fixes #12950