Preserve empty Worktrees slice when worktree list fails to load

If `git worktree list` fails, we want the Worktrees model to fall back
to an empty slice so callers iterating over it stay correct. The error
branch was setting it to `[]`, but the line below unconditionally
overwrote it with the nil `worktrees` value from the failed call.

Use an else branch so the empty-slice fallback actually sticks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>