[buildcff2vf] Don't return zero when input requires compatibilization

[josh-hadley]

Description

What the title says. Fixes #1667.

Checklist:

• [x] I have followed the Contribution Guidelines
• [x] I have ~added~ updated test code and data to prove that my code functions correctly
• [x] I have verified that new and existing tests pass locally with my changes
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] ~I have made corresponding changes to the documentation~

[frankrolf]

Thank you!