ci: allow null compare in check-yaml-fields script

Allow check for `None` type in check-yaml-fields.py script. It wasn't
clear from the original commit - 2ddde49 - if we purposefully didn't
want to compare `null` in the script, or it was just a typo. This will
now let us assert a field should be `null` instead of some other value,
and fail properly when it is not.

scripts/check-yaml-fields.py

@@ -27,8 +27,7 @@ def main():
                 v = v[index]
         if expected is None:
             print(v)
-        else:
-            assert v == expected, "{!r} != {!r}".format(v, expected)
+        assert v == expected, "{!r} != {!r}".format(v, expected)
 
 
 main()