Missing commit from e95

Created by: muffato

Description

I forgot to include this commit in #321 😞 The commit itself has already been reviewed and accepted on the release/95 branch (#312). This PR is to copy it to master before the e96 branch is created.

The change is a fix to handle undefined values when filtering whole-genome alignments

Testing

Have you added/modified unit tests to test the changes?

yes

If so, do the tests pass/fail?

yes