mgmt: allow functions to return None if no data is to be returned

Don't force `return ''` everywhere.

QubesOS/qubes-issues#2622

qubes/tools/qubesd.py

@@ -117,6 +117,7 @@ class QubesDaemonProtocol(asyncio.Protocol):
 
     def send_response(self, content):
         self.send_header(0x30)
+        if content is not None:
             self.transport.write(content.encode('utf-8'))
 
     def send_event(self, subject, event, **kwargs):