osmo-smdpp: Make sure to return empty HTTP response in handleNotification

SGP.22 is quite clear in that handleNotification shall return an empty
HTTP response body.  Let's make sure we comply to that and don't report
a JSON response.

Change-Id: I1cad539accbc3e7222bfd4780955b3b1ff694c5b

osmo-smdpp.py

@@ -186,7 +186,9 @@ class SmDppHttpServer:
             print("Rx JSON: %s" % json.dumps(content))
             set_headers(request)
 
-            output = func(self, request, content) or {}
+            output = func(self, request, content)
+            if output == None:
+                return ''
 
             build_resp_header(output)
             print("Tx JSON: %s" % json.dumps(output))