Handling routes with empty parameters

I am using a route with three dynamic parameters. The first is functional (the record id); the other two are for SEO purposes. One of the last two might have no value, resulting in the query string appearing like this:
/item/4868//scott-walter
… with a double forward slash. In this case the query fails. Is there an option to allow for empty parameters? I thought the ? suffix might do this, but I think this must mean the parameter itself is optional, not whether or not it contains a value.

If there isn’t an option for this, I’ll set a default to avoid empty parameters. I tried using:
{{author.default('no author')}}
… but this doesn’t return anything.

I used:
{{author?author:'no author'}}
… and it worked as expected.

There seems to be a problem using the ‘default’ formatter, unless I’m not using it correctly. (This is in App Connect; I haven’t had any problems using ‘default’ in Server Connect, in query builder.)