External links cannot have empty query parameters

Thanks for all you effort, but from my side this doesn't look like bug.
It looks like default behavior.
My suggestion if you want this behavior create custom formatter, or contrib module that implements this.

This is a duplicate of 🐛 Url generator will add '=' after query parameter when value is an empty string Needs work which is both older and has more recent work on a fix. Credit has been granted to the people who worked on patches.