`tt update` does not create reference images if `ref/` is empty

[ensko]

Description

If ref/ exists, the test should be treated as persistent. However, creating initial reference images (e.g. when converting compile-only to persistent) does not work as expected

Steps to Reproduce the Problem

1. prepare an ephemeral test case (no ref/)
2. add ref/ to make it persistent
3. run tt update

Expected Behavior

The ref/ directory is poulated with images

Actual Behavior

The ref/ directory remains empty and "1 skipped" appears in the output

Additional Context

• Platform: Linux x64
• Version: 0.3.3