It produced a duplicate entry when listing the extractors with '--list-extractors' and generates noise in the commit log when generating the supported sites webpage (like in 09f355f73b)
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".