Only add node to the log file when needed

We were adding an empty node to the log file when run in a single-node
setup.  Add a test and don't add a node instead.

Change-Id: I9c579140705adb7e85aaf3e17e1dea75bf75776d
diff --git a/aos/configuration.cc b/aos/configuration.cc
index 65c218e..46f07ac 100644
--- a/aos/configuration.cc
+++ b/aos/configuration.cc
@@ -603,7 +603,10 @@
 }
 
 const Node *GetNode(const Configuration *config, std::string_view name) {
+  CHECK(config->has_nodes())
+      << ": Asking for a node from a single node configuration.";
   for (const Node *node : *config->nodes()) {
+    CHECK(node->has_name()) << ": Malformed node " << FlatbufferToJson(node);
     if (node->name()->string_view() == name) {
       return node;
     }