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
---
M osmo-smdpp.py
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/osmo-smdpp.py b/osmo-smdpp.py
index 6c275ac..390db18 100755
--- a/osmo-smdpp.py
+++ b/osmo-smdpp.py
@@ -186,7 +186,9 @@
             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))

