Move node() == nullptr check into //aos:configuration

This simplifies the code surprisingly significantly for dealing with
single node systems.

Change-Id: Ib317e16a9055fbf1341a194120a5b34f05b2d8f8
diff --git a/aos/configuration.cc b/aos/configuration.cc
index 112c567..4a98297 100644
--- a/aos/configuration.cc
+++ b/aos/configuration.cc
@@ -643,10 +643,17 @@
 }
 
 bool ChannelIsSendableOnNode(const Channel *channel, const Node *node) {
+  if (node == nullptr) {
+    return true;
+  }
   return (channel->source_node()->string_view() == node->name()->string_view());
 }
 
 bool ChannelIsReadableOnNode(const Channel *channel, const Node *node) {
+  if (node == nullptr) {
+    return true;
+  }
+
   if (channel->source_node()->string_view() == node->name()->string_view()) {
     return true;
   }