The clover-plugin Ratio.getPercentageFloat() does not seem to handle a ratio of N / 0 properly. It seems that this should be 100%, not 0%.
It looks like https://github.com/jenkinsci/clover-plugin/commit/ddbf016c17933cba373903bb797f18172b422509 may have introduced this issue.
I have created a pull on GitHub (https://github.com/jenkinsci/clover-plugin/pull/9).
I have also added some tests for the Ratio.getPercentageFloat() in my pull request, but they are incomplete due to not being able to find documentation on the edge cases.
I am willing to change the way the patch is assembled (if you want the testing broken into a different branch pull request, for example). I am also willing to flesh out the test for that method, I just need to know what the expected behavior should be.