Theta star planner crashes

[daisukes]

Hello, I have been trying to use the theta star planner, which is very fast, and I love it very much!!
I found a bug that caused a crash when the planner could not make a plan (return zero-length poses).
Please let me know if you want to make a PR (or two for galactic/main branches)

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04
• ROS2 Version:
  • galactic binary release
• Version or commit hash:
  • galactic 3ac8819
• DDS implementation:
  • Fast-RTPS (does not matter)

Steps to reproduce issue

• provide a goal where the theta star planner cannot produce a plan

Expected behavior

• fail without a crash

Actual behavior

• crashes at theta_star_planner.cpp#L119
• length of global_path.poses can be zero
  https://github.com/ros-planning/navigation2/blob/d8f50da2ed59879112105ac1488e9139ee7f3c71/nav2_theta_star_planner/src/theta_star_planner.cpp#L118-L119

[SteveMacenski]

Seems like a simple fix! Can you submit a PR to check if the size is 0 and if so, return early? No need for any of that orientation logic if its completely empty. Just main is fine, it'll be backported to galactic next time we do a sync