[wiring] Mesh: sanity checks in publish/subscribe, whitespacing

Author: avtolstoy

wiring/inc/spark_wiring_mesh.h

@@ -126,7 +126,7 @@ class MeshPublish {
         // and system.
     }
 
-    int publish(const char* topic, const char* data);
+    int publish(const char* topic, const char* data = nullptr);
 
     int subscribe(const char* prefix, EventHandler handler);