fix: fix nil point about Outputs.ExitCode. Fixes #7458 (#7459)

Signed-off-by: FengyunPan2 <pan_feng_yun@163.com>

workflow/controller/operator.go

@@ -1547,7 +1547,7 @@ func buildRetryStrategyLocalScope(node *wfv1.NodeStatus, nodes wfv1.Nodes) map[s
 	lastChildNode := getChildNodeIndex(node, nodes, -1)
 
 	exitCode := "-1"
-	if lastChildNode.Outputs != nil {
+	if lastChildNode.Outputs != nil && lastChildNode.Outputs.ExitCode != nil {
 		exitCode = *lastChildNode.Outputs.ExitCode
 	}
 	localScope[common.LocalVarRetriesLastExitCode] = exitCode