fix: best checkpoint not found issue

[Mai0313]

What does this PR do?

This PR addresses the issue where Lightning saves "best ckpt" with absolute paths in each checkpoint. These are not copied during fine-tuning or continue training, causing issues if the "best ckpt" is deleted or moved.

With this change, the system will use current weights for testing if the "best ckpt" is not found, ensuring smooth execution of fine-tuning and continue training stages.

The changes made are as follows:

if ckpt_path == "":
    log.warning("Best ckpt not found! Using current weights for testing...")
    ckpt_path = None

to

if not ckpt_path:
    log.warning("Best ckpt not found! Using current weights for testing...")
    ckpt_path = None

Fixes #621

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