this change is created in the same spirit of 0104c7d3, which used
`std::map::contains()` in the place of `std::map::count()` when
checking for the existence of a paramter with given name for better
readability.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#21158