fix(ai): handle empty combat withdrawal members

[matthewevans]

Fixes #7095.\n\nAvoids eager indexing when combat-withdrawal candidate members are empty or non-singular; only a one-member slice yields a combat pair.

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[matthewevans]

Superseded by #7270, which merged the same single_member safety fix into main (commit c22f9c08c6). Closing this draft because its base branch was not main.