Fix: _labels_for_format was returning None labels

[mskrajnowski]

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:

[bumpversion:file:bower.json]
parse = "version": "(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)"
serialize = "version": "{major}.{minor}.{patch}"

[peritus]

Needs regression test case demonstrating the bug.

[peritus]

Thanks for the pull request, you seem to have a valid case here.

Also thanks for pointing out that I didn't mention the search/replace options in the README. Using these, you should be able to simplify your config as follows (assuming you want to inherit the {major}.{minor}.{patch} defaults):

[bumpversion:file:bower.json]
search = "version": "{current_version}"
replace = "version": "{new_version}"