init: only print backtraces on subsequent reboots with a value

Userspace reboot resets sys.powerctl to an empty string once it
starts, which was inadvertently triggering the backtrace of the main
init thread.

Test: no more unexpected backtraces
Change-Id: I35d6f1b37aa31a46ae9266647f41a709f28b6099
diff --git a/init/property_service.cpp b/init/property_service.cpp
index 1e4e127..655b8de 100644
--- a/init/property_service.cpp
+++ b/init/property_service.cpp
@@ -489,7 +489,9 @@
         }
         LOG(INFO) << "Received sys.powerctl='" << value << "' from pid: " << cr.pid
                   << process_log_string;
-        DebugRebootLogging();
+        if (!value.empty()) {
+            DebugRebootLogging();
+        }
     }
 
     // If a process other than init is writing a non-empty value, it means that process is