Prevent division by zero

[Berehum]

Please describe what your PR is fixing
In the statistics there are a few divisions where it would be possible to divide by zero. I've implemented a ternary operator to prevent divisions by zero.

Concretely, which issues does your PR solve? (Please reference them by typing Fixes/References Inter-Actief/amelie#<issue_id>)
Fixes/References #892

Does your PR change how we process personal data, impact our privacy document, or modify (one of) our data export(s)?
no

Does your PR include any django migrations?
no

Does your PR include the proper translations (did you add translations for new/modified strings)?
no, my PR does not include translations

Does your PR include CSS changes (and did you run the compile_css.sh script in the scripts directory to regenerate the compiled.css file)?
no, my PR does not include CSS changes

Does your PR need external actions by for example the System Administrators? (Think about new pip packages, new (local) settings, a new regular task or cronjob, new management commands, etc.)?
no

Did you properly test your PR before submitting it?
yes