Another hole found ahead of enabling `post_error_fails_task`/`teardown_task_can_fail_task` broadly.

`evergreen/failed_unittests_gather.sh`'s `while read ... done <<<"${core_files}"` always ran its body at least once, even when `core_files` was empty (a here-string on an empty var still feeds one empty line to `read`). With `gdb` present and an empty `$core_file`, the pipeline feeding `binary_file=$(...)` returns nonzero, and `set -e`/`pipefail` aborted the script — on every green unit-test task, not just failures. This was silently swallowed until teardown failures can fail the task.

Fix: skip the loop entirely when there are no core files.