In this case, it looks rather harmless (since the conditions for --restrict-filenames should not change while a process is running), but just to be sure.
This also simplifies the interface for callers, who can just pass in the idiomatic None for "I don't care, whatever is the default".
If it sets the 'restrictfilenames' param, it will first report a warning. It will try to get the logger from the 'params' property, which would be set at that moment to None, raising the error 'AttributeError: 'NoneType' object has no attribute 'get''