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>

pkg/gui/controllers/helpers/refresh_helper.go

@@ -724,9 +724,9 @@ func (self *RefreshHelper) loadWorktrees() {
 	if err != nil {
 		self.c.Log.Error(err)
 		self.c.Model().Worktrees = []*models.Worktree{}
+	} else {
+		self.c.Model().Worktrees = worktrees
 	}
-
-	self.c.Model().Worktrees = worktrees
 }
 
 func (self *RefreshHelper) refreshWorktrees() {