Ignore node specific maps when no node is specified

In some contexts, we do a channel lookup without a node.  This is
especially true in LogReader when we are looking to register a mutation
callback for a message.  In this case, we were matching node specific
maps when we shouldn't be.  The end result is that subscribing to `/aos`
was resolving to `/pi2/aos` since that was the first remap found.

Change-Id: I428a01d24c7576ad9588b48ffbfb245d4bd98994
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/configuration_test.cc b/aos/configuration_test.cc
index 64b286e..da96402 100644
--- a/aos/configuration_test.cc
+++ b/aos/configuration_test.cc
@@ -240,6 +240,9 @@
   EXPECT_THAT(GetChannel(config, "/batman", ".aos.bar", "app1", pi1),
               aos::testing::FlatbufferEq(ExpectedMultinodeLocation()));
 
+  // Tests that node specific maps get ignored
+  EXPECT_EQ(GetChannel(config, "/batman", ".aos.bar", "", nullptr), nullptr);
+
   // Tests that a root map/rename fails with a node specific map for the wrong
   // node.
   EXPECT_EQ(GetChannel(config, "/batman", ".aos.bar", "app1", pi2), nullptr);