Fix a crash when missing rack id (#172)

netbox_agent/server.py

@@ -66,7 +66,6 @@ class ServerBase():
 
     def update_netbox_location(self, server):
         dc = self.get_datacenter()
-        rack = self.get_rack()
         nb_rack = self.get_netbox_rack()
         nb_dc = self.get_netbox_datacenter()
 
@@ -79,7 +78,11 @@ class ServerBase():
             update = True
             server.site = nb_dc.id
 
-        if rack and server.rack and server.rack.id != nb_rack.id:
+        if (
+            server.rack
+            and nb_rack
+            and server.rack.id != nb_rack.id
+        ):
             logging.info('Rack location has changed from {} to {}, updating'.format(
                 server.rack,
                 nb_rack,