Fix nil pointer dereference when ClusterGroup/Group is used #30695

Job	Run time
Check whether tests need to be run based on diff	15s
Build Flow Aggregator image to be used for Kind e2e tests	3m 8s
Build Antrea image to be used for Kind e2e tests	4m 2s
E2e tests on a Kind cluster on Linux (noEncap)	37m 4s
E2e tests on a Kind cluster on Linux	45m 22s
API compatible with client version N-1	8m 59s
E2e tests on a Kind cluster on Linux with non default values (proxyAll=true, LoadBalancerMode=DSR...	46m 17s
E2e tests on a Kind cluster on Linux with all features enabled	55m 39s
E2e tests on a Kind cluster on Linux for Flow Visibility	31m 22s
E2e tests on a Kind cluster on Linux (hybrid)	37m 10s
NetworkPolicy conformance tests on a Kind cluster on Linux	8m 10s
Upgrade from Antrea version N-1	8m 47s
Upgrade from Antrea version N-2	8m 48s
API compatible with client version N-2	8m 51s
Validate metrics in Prometheus document match running deployment's	1m 57s
Antrea-native (VLAN) secondary network tests on a Kind cluster on Linux	2m 55s
Delete uploaded images	3s
	5h 8m 49s

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