fix(RELEASE-2646): fix empty pids array

Access to empty `pids` cannot be done via "${#pids[@]}"
as that's causing `unbound variable` error.
Instead it has to be access as "${#pids[@]:-0}"

Assited-By: claude
Signed-off-by: Jindrich Luza <jluza@redhat.com>

Author: midnightercz

integration-tests/pipelines/e2e-tests-periodic-pipeline.yaml

@@ -181,7 +181,7 @@ spec:
 
               # Cap parallel suites to limit kubectl/API and memory spikes (step memory: 6Gi).
               MAX_PARALLEL="$(params.MAX_PARALLEL)"
-              declare -A pids
+              declare -A pids=()
               testcase_index=0
 
               reap_finished_test_jobs() {