Improve unit test handling in CI after migration. (#67217)

* Improve unit test handling in CI after migration.

* Fix undefined var in bash 4.3 and earlier.

See https://git.savannah.gnu.org/cgit/bash.git/tree/CHANGES?id=3ba697465bc74fab513a26dea700cc82e9f4724e#n878

test/utils/shippable/units.sh

@@ -44,17 +44,27 @@ networks=(
 )
 
 for network in "${networks[@]}"; do
-    group1+=(--exclude "test/units/modules/network/${network}/")
-    group2+=("test/units/modules/network/${network}/")
+    test_path="test/units/modules/network/${network}/"
+
+    if [ -d "${test_path}" ]; then
+        group1+=(--exclude "${test_path}")
+        group2+=("${test_path}")
+    fi
 done
 
 case "${group}" in
-    1) options=("${group1[@]}") ;;
-    2) options=("${group2[@]}") ;;
+    1) options=("${group1[@]:+${group1[@]}}") ;;
+    2) options=("${group2[@]:+${group2[@]}}") ;;
 esac
 
+if [ ${#options[@]} -eq 0 ] && [ "${group}" -eq 2 ]; then
+    # allow collection migration unit tests for group 2 to "pass" without updating shippable.yml or this script during migration
+    echo "No unit tests found for group ${group}."
+    exit
+fi
+
 ansible-test env --timeout "${timeout}" --color -v
 
 # shellcheck disable=SC2086
 ansible-test units --color -v --docker default --python "${version}" ${COVERAGE:+"$COVERAGE"} ${CHANGED:+"$CHANGED"} \
-    "${options[@]}" \
+    "${options[@]:+${options[@]}}" \