Prevent invalid accesses on paths in path_longer_on_approach

[lss0815]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses
Primary OS tested on	(Ubuntu)
Robotic platform tested on	(Custom Robot)
Does this PR contain AI generated software?	(No)

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Description of contribution in a few bullet points

• We encountered a segmentation fault in the behavior tree while operating our robot. Upon investigation, we discovered that the PathLongerOnApproach node was causing these segmentation faults.
• When the ComputePathToPoseAction sets an empty path in the behavior tree port path due to failure or cancellation, the path from PathLongerOnApproach can be empty. Without checking its size, the node encounters segmentation faults at that time.
• Co-author: @JinyongJeong

Description of documentation updates required from your changes

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Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[SteveMacenski]

Thanks @lss0815!