fix: prevent division by zero in charge corrections

[diogomart]

Description

Prevent division by zero when all charges are zero in code that makes sure that partial charges sum to an integer.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist:

Please go through the following checklist before submitting the PR:

• Written a description of the feature or bug fix above.
• Ran pytest locally and all tests have passed.
• If applicable, documentation was updated with new feature.
• Docstring included for any new classes/functions/methods, or for significantly modifed existing functions.
• (Optional) new test added to account for new feature.