Correct check for _sdr in command

In the attempt to clean up, one change broke how a command
object knows whether it has an SDR yet or not.  Correct that
mistake.

Change-Id: I76faaccf15c2dbfa2b7d5a3a4e1665e0cefe4c6d

pyghmi/ipmi/command.py

@@ -312,7 +312,7 @@ class Command(object):
 
         :returns: Iterator of sdr.SensorReading objects
         """
-        if '_sdr' not in self.__dict__:
+        if self._sdr is None:
             self._sdr = sdr.SDR(self)
         for sensor in self._sdr.get_sensor_numbers():
             rsp = self.raw_command(command=0x2d, netfn=4, data=(sensor,))