[BtActionServer] default empty search_directories in constructor

[doisyg]

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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	Dexory sim
Does this PR contain AI generated software?	No
Was this PR description generated by AI software?	No

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Description of contribution in a few bullet points

In relation to #5426 (comment)
Make search_directories optional to not break external use of BtActionServer (without a migration step).
cc @Jad-ELHAJJ

Description of documentation updates required from your changes

Description of how this change was tested

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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
• Should this be backported to current distributions? If so, tag with backport-*.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
...ee/include/nav2_behavior_tree/bt_action_server.hpp	83.33% <ø> (ø)
...clude/nav2_behavior_tree/bt_action_server_impl.hpp	88.82% <ø> (ø)
...core/include/nav2_core/behavior_tree_navigator.hpp	89.28% <ø> (ø)

... and 4 files with indirect coverage changes

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[SteveMacenski]

I ack this - but also this is actually a neat feature. I’d recommend checking it out! You can call actions by a tree’s name rather than by filepath!