Fix not-found response

mocha_server.py

@@ -31,6 +31,9 @@ def fetch_user(user_id):
     output = json.dumps([dict(row) for row in output])
     conn.close()
 
+    if output == '[]':
+        output = None
+
     return output
 
 
@@ -44,12 +47,11 @@ def process_request(uri):
     parts = uri.split('/')[1:]
     assert parts[0] == 'mocha'
 
-    output = ''
+    output = None
 
     if parts[1] == 'users':
         output = fetch_user(parts[2])
 
-
     return output