added default values to parameter declaration

[adityakamath]

parameter declaration without default values has been deprecated. This update makes it work for ROS2 Galactic, as well as Foxy (I haven't tested this but it should not cause an impact)

Not sure if a separate branch is needed for Galactic, as this is a minor change.

[geoporus1]

I checked the changes and they look good! for the sake of cleanliness and good tracking for each of the ROS2 releases, a new branch for Galactic should probably be created, but for now, I think just the one, for foxy, will still do well enough, as there are no other significant changes required between the 2.

[naoki-mizuno]

Thanks for your contribution!