Fix: use empty strings explicitly for path and sha

[phtea]

When using
:lua require('telescope').extensions.git_worktree.git_worktrees()
A warn message occurs and picker does not function normally (doesn't pick first element by default and doesn't show all elements)

Invalid 'hl_group': Expected Lua string

This change sets "" (empty string) as a fallback for non-highlighted fields to avoid undefined behaviour.