prevent presenting NaN on the page

What does this PR do? How does it affect users?

prevent presenting NaN as a percentage value

Step through the code line by line. Things to keep in mind as you review:

Are there any edge cases not covered by this code?
Does this code follow conventions (naming, formatting, modularization, etc) where applicable?

Fetch the branch and/or deploy to staging to test the following:

[ ] Does the code compile without warnings (check shell, console)?
[ ] Do all tests pass?
[ ] Does the UI, pixel by pixel, look exactly as expected (check various screen sizes, including mobile)?
[ ] If the feature makes requests from the browser, inspect them in the Web Inspector. Do they look as expected (parameters, headers, etc)?
[ ] If the feature sends data to Keen, is the data visible in the project if you run an extraction (include link to collection/query)?
[ ] If the feature saves data to a database, can you confirm the data is indeed created in the database?