fix serial cmd

raspberry/roberto/Serial.py

@@ -50,7 +50,7 @@ class Serial(object):
                     print(data)
                     command = data[0]
                     argument = data[2:].strip()
-                    if command.isalnum() and command in self._callbacks:
+                    if command and command in self._callbacks:
                         cb = self._callbacks[command]
                         if cb:
                             cb(argument)