grpc-js-xds: XdsClusterImpl: fix handling of non-complete pick result type

Author: murgatroid99

packages/grpc-js-xds/src/load-balancer-xds-cluster-impl.ts

@@ -173,7 +173,7 @@ class XdsClusterImplPicker implements Picker {
       return {
         pickResultType: originalPick.pickResultType,
         status: originalPick.status,
-        subchannel: pickSubchannel?.getWrappedSubchannel() ?? null,
+        subchannel: pickSubchannel?.getWrappedSubchannel?.() ?? null,
         onCallStarted: () => {
           originalPick.onCallStarted?.();
           pickSubchannel?.getStatsObject()?.addCallStarted();