Comment dummy TURN parameters in default config

This default config is parsed and used a base before the actual
config is overlaid, so with these values not commented out, the
code to detect when no turn params were set and refuse to generate
credentials was never firing because the dummy default was always set.

changelog.d/3511.bugfix

@@ -0,0 +1 @@
+Don't generate TURN credentials if no TURN config options are set

synapse/config/voip.py

@@ -30,10 +30,10 @@ def default_config(self, **kwargs):
         ## Turn ##
 
         # The public URIs of the TURN server to give to clients
-        turn_uris: []
+        #turn_uris: []
 
         # The shared secret used to compute passwords for the TURN server
-        turn_shared_secret: "YOUR_SHARED_SECRET"
+        #turn_shared_secret: "YOUR_SHARED_SECRET"
 
         # The Username and password if the TURN server needs them and
         # does not use a token