Fix: _labels_for_format was returning None labels

If there were any characters after the last label in the serialize string, `_labels_for_format` would return an extra `None` label.

Example config file:
```cfg
[bumpversion:file:bower.json]
parse = "version": "(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)"
serialize = "version": "{major}.{minor}.{patch}"
```