msggen: Add stop method to generators

We'll need this when testing the grpc interface in pyln-testing,
otherwise tests just slowly die and wither.

contrib/pyln-testing/pyln/testing/grpc2py.py

@@ -850,3 +850,8 @@ def signmessage2py(m):
         "recid": hexlify(m.recid),  # PrimitiveField in generate_composite
         "zbase": m.zbase,  # PrimitiveField in generate_composite
     })
+
+
+def stop2py(m):
+    return remove_default({
+    })