test: cover worktree_base empty and spaced WT_BASE_DIR

[RodrigoEspinosa]

🎯 What: Improved test coverage for worktree_base bash function in tests/worktree_base.bats.
📊 Coverage: Added test coverage for WT_BASE_DIR explicitly set to an empty string, triggering fallback to parent of repo_root. Added coverage for WT_BASE_DIR containing spaces in the directory path to make sure spaces are correctly supported.
✨ Result: Better unit testing of our worktree path resolution logic with edge case safety nets.

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PR created automatically by Jules for task 3809859394771532259 started by @RodrigoEspinosa

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