pkg/payload/task_graph: Require firstIncompleteNode to have tasks

Our graph has some nodes without tasks, e.g. nodes which create a
choke point between two sets of parallel nodes.  Ideally we give all
nodes descriptive names [1], but until we have that, require at least
one entry in Tasks before we store a node as firstIncompleteNode.
This avoids the chance of panicking during the subsequent [2]:

  fmt.Errorf("%d incomplete task nodes, beginning with %s", incompleteCount, firstIncompleteNode.Tasks[0])

[1]: #435
[2]: https://bugzilla.redhat.com/show_bug.cgi?id=1903382

Author: wking

pkg/payload/task_graph.go

@@ -536,7 +536,7 @@ func RunGraph(ctx context.Context, graph *TaskGraph, maxParallelism int, fn func
 	incompleteCount := 0
 	for i, result := range results {
 		if result == nil {
-			if firstIncompleteNode == nil {
+			if firstIncompleteNode == nil && len(graph.Nodes[i].Tasks) > 0 {
 				firstIncompleteNode = graph.Nodes[i]
 			}
 			incompleteCount++