Stop bidirectional A* when either frontier is empty

[dhairyajangir]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Stop the bidirectional A* loop when either frontier is empty. The previous or condition allowed another iteration with one empty list, although that iteration unconditionally pops from both frontiers.

For example, with grid = [[0, 1, 0], [1, 1, 0]], start (0, 0), and goal (0, 2), the forward frontier empties first and the current implementation raises IndexError: pop from empty list. The change returns the existing start-only no-path result. The mirrored case, where the backward frontier empties first, is fixed as well.

This is a one-line guard change. Node matching, heuristics, and the existing return convention are unchanged. Existing doctests are preserved as requested by the contribution guidelines.

Validation on free-threaded CPython 3.15.0rc2:

• All 18 existing doctests pass.
• Separate regression checks cover each frontier emptying first, both emptying together, and a reachable three-cell corridor: baseline 2 pass / 2 fail; fixed 4 pass.
• pre-commit run --all-files --show-diff-on-failure passes all applicable configured hooks.

AI assistance was used to prepare this change; a separate agent review reproduced the failures and verified the fix and unchanged controls.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

No new file or algorithm is added, and no issue is claimed. The existing class doctests pass; this change adds no doctests or new functions.