Handle failed LQR local paths in LQR-RRT* steering

[huxiaoyi-ovo]

Summary

• handle an empty local path returned by the LQR planner
• return None from LQRRRTStar.steer() instead of indexing an empty list
• add a deterministic regression test using the real LQR failure path

Problem

LQRPlanner.lqr_planning() explicitly returns ([], []) when it cannot find a path. LQRRRTStar.steer() currently checks only if px is None, so an empty list passes the check and the subsequent px[-1] / py[-1] access raises IndexError.

Fix

Treat an empty sampled path as a failed steering attempt with if not px: return None. This matches the existing planner convention that None indicates steering failure and lets the RRT* search continue safely.

Tests

Added test_steer_handles_failed_local_planner. The test uses the real LQRPlanner with MAX_TIME = 0.0, which deterministically exercises its existing no-path return ([], []), and verifies that steer() returns None without raising.